WO1998028467A1

WO1998028467A1 - Method of coating complex-shaped ceramic-metal composites and the products produced thereby

Info

Publication number: WO1998028467A1
Application number: PCT/US1997/021896
Authority: WO
Inventors: Mani Shabrang; Bonnie S. Pelon
Original assignee: The Dow Chemical Company
Priority date: 1996-12-20
Filing date: 1997-11-26
Publication date: 1998-07-02

Abstract

The invention is a process for preparing a coated ceramic-metal composite article with few or no defects in its surface(s), comprising: (a) forming a first smoothing layer(s) on one or more face(s) of a shaped ceramic-metal composite article; and (b) further forming a plating layer on the first smoothing layer(s) on one or more face(s) of the shaped ceramic-metal composite, such that a coated ceramic-metal composite article comprising one or more metal phases and one or more ceramic phases is formed, wherein the article has no defects in the plating layer. The invention is also a coated ceramic-metal composite article with a first smoothing layer(s) and a plating layer on the first smoothing layer on one or more face(s) of the ceramic-metal composite article with no defects of more than 100 micrometers on the surface of the article. The process of the invention allows the preparation of a coated ceramic-metal composite article with no defects on the surface(s) of the article after the coating process. A coated ceramic-metal composite article is prepared which contains no defects on the surface(s) of the article after the coating process.

Description

METHOD OF COATING COMPLEX-SHAPED CERAMIC-METAL COMPOSITES AND THE

PRODUCTS PRODUCED THEREBY

This invention relates to a process for preparing a coated ceramic-metal composite article with few or no surface defects and the products produced thereby.

Ceramics are typically known as low-density materials with high hardness and stiffness; however, their brittleness limits their usefulness. Furthermore, ceramics are typically formed by creating a densified compact that requires significant and expensive grinding to achieve a final shape due to the large amount of shrinkage that occurs during densification of the compact. Metals are typically non-brittle, non-breakable materials; however, they lack some of the desirable properties of the ceramics, such as high hardness and stiffness. Therefore, combining a ceramic with a metal can create a composite material that exhibits the properties of a ceramic and a metal.

Processes for making ceramic-metal composite articles using ceramic preforms are known in the art. PCT/US95/15794 discloses a ceramic-metal substrate and a process for making ceramic-metal substrates involving forming a ceramic preform, infiltrating the preform with a metal and coating the infiltrated substrate with a plating layer. It would be desirable to achieve an even smoother, defect-free plating layer.

What is needed is a process for preparing a coated ceramic-metal composite article with no surface defects in the plating layer. What is needed is a coated ceramic-metal composite article with no defects on its surface(s) after plating.

The invention is a process for preparing a coated ceramic-metal composite article with few or no defects in its surface(s), comprising:

(a) forming a first smoothing layer(s) on one or more face(s) of a shaped ceramic-metal composite article; and

(b) further forming a plating layer on the first smoothing layer(s) on one or more face(s) of the shaped ceramic-metal composite such that a coated ceramic-metal composite article comprising one or more metal phases and one or more ceramic phases is formed, wherein the article has few or no defects in the plating layer of 100 micrometers or greater.

The invention is also a coated ceramic-metal composite article with a first smoothing layer(s) and a plating layer on the first smoothing layer on one or more face(s) of the ceramic-metal composite article with no defects of more than 100 micrometers on the surface of the article.

The process of the invention allows the preparation of a coated ceramic-metal composite article with few or no defects on the surface(s) of the article after the coating process. A coated ceramic-metal composite article is prepared which contains no defects on the surface(s) of the article after the coating process.

The coated ceramic-metal composite articles of the invention are articles with no defects on the surface(s) of the coated article of 100 micrometers or greater. Defects in the surface of the uncoated ceramic-metal composite articles are caused in part by nonuniform infiltration of the metal into the ceramic material. The nonuniform infiltration leads to areas on the surface of the ceramic substrate receiving different levels of the metal during infiltration, thus allowing excess metal to remain on the surface of the ceramic-metal substrate. Also, undesirable reaction phases form at all the points of infiltration on the ceramic-metal substrate. The undesirable reaction phases are chemically unstable and react with air to degrade, causing a defect in the surface of the substrate or the phases can cause pullout damage to the surface of the infiltrated part upon machining. Pullout damage results from machining of the excess metal or undesirable phase on the surface of the article with partial removal of the undesirable phase occurring which leads to pitting and defects in the surface of the article. Therefore, if infiltration occurs at several points on the surface of the ceramic substrate and/or is nonuniform, the finishing costs of the final ceramic-metal substrate are higher due to the regions of excess metal and undesirable phases formed on or within the surface(s) of the composite.

The process of this invention allows a smoothing material to form a first smoothing layer(s) on the surface of the ceramic-metal composite article thus allowing later coatings or plating layers to fill in some or all of the these surface defects. The smoothing layer refers to a layer which allows lateral and vertical growth of any subsequent coating deposited on and within the ceramic-metal composite article. The smoothing layer allows any subsequent plating or coating layer to fill the defects on and within the surface of the ceramic-metal substrate. The surface defects, as stated above, are caused by nonuniform infiltration of the metal with the ceramic substrate and the machining necessitated by the uneven infiltration. By forming a first smoothing layer on one or more face(s) of the substrate, the plating layer can be formed on the first smoothing layer on the coated article to form a relatively defect-free coated surface. A ceramic-metal composite article without the first smoothing layer(s) has surface defects which can be from 50 micrometers or greater in depth. A surface defect is measured from the planar surface of the ceramic-metal substrate to the actual depth of the defect with the ceramic-metal substrate. This is unacceptable for usage in many applications such as computer disk drive components. Preferably, a coated ceramic-metal composite article with a first smoothing layer(s) and a plating layer thereon has no surface defects which average 100 micrometers or greater, more preferably 50 micrometers or greater and even more preferably 10 micrometers or greater as measured on a WYKO Surface Interferometer. Furthermore, for specific computer component applications, the industry method of measuring surface defects is by visually inspecting the surface. If no defects are observable to the naked eye, the computer component is pronounced defect- free. A person with normal visual acuity can observe a defect of 100 micrometers or greater with a one-foot distance between the naked eye and the object being observed. A person with exceptional visual acuity can observe defects of 50 micrometers or greater with a one- foot distance between the naked eye and the object being observed.

Preferably, the process can be used to prepare coated ceramic-metal composite articles of complex shape. The process of the invention is used to prepare coated ceramic-metal composite articles of complex-shape comprising one or more metal phases and one or more ceramic phases. The ceramic-metal composite material of the coated complex-shaped ceramic-metal composite article preferably comprises at least three phases. Preferably, each of the phases is present in an amount of at least 2 volume percent based on the volume of the multi-phased ceramic-metal material. The ceramic-metal composite material of the coated ceramic-metal composite article preferably has a residual- free metal content of 2 volume percent or greater. The ceramic-metal composite article preferably has a residual-free metal content of 75 volume percent or less, more preferably 50 volume percent or less and even more preferably 25 volume percent or less.

Preferably, the coated ceramic-metal composite article has a theoretical density of 85 percent or greater, more preferably 98 percent or greater, and even more preferably 99.5 percent or greater, wherein the theoretical density is 100 times the ratio of the final measured part density over the theoretical density of the material with no porosity. The coated ceramic-metal composite articles preferably have an elastic modulus high enough to prevent or reduce warping, sagging, fluttering or resonating during handling and use. Preferably, the ceramic-metal composite article demonstrates an elastic modulus of 100 GPa or greater, more preferably 150 GPa or greater, and even more preferably 200 GPa or greater.

The coated complex-shaped ceramic-metal composite articles of the invention preferably demonstrate flexure strength high enough to impart shock resistance and resistance to damage during handling and usage. The coated ceramic-metal composite articles of the invention preferably demonstrate a flexure strength of 250 MPa or greater, more preferably 350 MPa or greater and even more preferably 450 MPa or greater. If electrical resistivity is a desired property, the coated ceramic-metal composite articles of the invention preferably have an electrical resistivity low enough to prevent a build-up of static electricity. Preferably, if low electrical resistivity is a desired property, the coated composite article of the invention demonstrates an electrical resistivity of 10² ohm-cm or less, more preferably 10^"4 ohm-cm or less, and even more preferably 10^'5 ohm-cm or less.

Ceramic-metal composite articles of this invention can be any complex- shaped part. The complex-shaped ceramic-metal composite articles of the invention are preferably computer disk drive components. Preferably, the articles are computer hard disks, actuators, sliders, load beams, support arms, actuator bearings, spacers, clamps, spindles, ball bearings, thrust bearings, journal bearings, base plates, housings or covers. More preferably, the articles are computer hard disks.

The process of the invention involves a series of steps to be performed in order to achieve a coated ceramic-metal composite article of complex shape, comprising one or more metal and one or more ceramic, with few or no surface defects in the coating. In order to prepare a coated ceramic-metal composite article, a selected ceramic is formed into a desired article shape. The preparation of the ceramic substrate as the basis for the ceramic-metal article can be accomplished by a variety of ceramic forming processes as discussed hereinafter. A metal is then infiltrated into the ceramic substrate. If desired, after infiltration, a heat-treatment may be performed to impart certain other mechanical properties to the complex-shaped ceramic-metal composite article. After infiltration and heat-treatment, the infiltrated body may optionally be machined and polished into a final desired shape. A first smoothing layer of a smoothing material is then formed on one or more face(s) of the complex-shaped ceramic-metal composite article. The smoothing material can be contacted with the complex-shaped ceramic-metal composite article by any means which results in the formation of a first smoothing layer of the smoothing material on one or more face(s) of the complex-shaped ceramic-metal composite article. Finally, a plating layer is formed on the first smoothing layer on one or more face(s) of the ceramic-metal composite article to form a defect-free surface coating on the article.

The metals useful for infiltrating the shaped ceramic substrate are selected based on their capability of chemically reacting or wetting with a chosen ceramic material at elevated temperatures such that the metal penetrates into the pores of the ceramic. Selected metals can be taken from Groups HA, IVB, VB, VIB, VIII, IIIA and IVA using the CAS notation of the Periodic Table as published in the Handbook of Chemistry and Physics. CRC Press, New York, New York, U.S.A. (1995-1996), and alloys thereof. Preferably metals for use herein include silicon, magnesium, aluminum, titanium, vanadium, chromium, iron, copper, nickel, cobalt, tantalum, tungsten, molybdenum, zirconium, niobium or mixtures and alloys thereof. Aluminum and alloys thereof are preferred because they exhibit high toughness, good electrical conductivity and machinability and have good wettability with a chosen ceramic, such as boron carbide, for example. Aluminum is best employed as an alloy which provides improved stiffness relative to pure aluminum. Alloys of aluminum with one or more of Cu, Mg, Si, Mn, Cr, or Zn are preferred. Alloys such as Al-Cu, Al-Mg, Al-Si, Al-Mn-Mg and Al-Cu-Mg-Cr-Zn and mixtures thereof are more preferred. Examples of such alloys are 6061™ alloy, 7075™ alloy, and 1350™ alloy, all available from The Aluminum Company of America, Pittsburgh, Pennsylvania.

The ceramics useful in this invention are chosen based on their chemical reactivity with the chosen metal at elevated temperatures so as to increase the penetration of the metal into the pores of the ceramic. Preferable ceramics for use herein include borides, oxides, carbides, nitrides, suicides or combinations thereof. Examples of combinations of ceramics include boron carbides, oxynitrides, oxycarbides and carbonitrides. More preferred ceramics are boron carbides, silicon carbides, titanium diborides and silicon nitrides. Even more preferred ceramics are BC, AIB₁₂, SiB₆ or SiB₄. A most preferred ceramic material is boron carbide because it has a desirable low density and high stiffness along with excellent wetting characteristics when in contact with a selected metal. The ceramic material used to form the shaped ceramic body is preferably in powder form and typically contains metal chemically bonded to the boron, oxygen, carbon, nitrogen or silicon of the ceramic. The powdered ceramics are preferably crystalline materials having grains that are 0.1 micrometers (0.1 x 10^'3 mm) or greater. The powdered ceramics are preferably crystalline materials having grains that are 50 micrometers (50 x 10^'3 mm) or less, more preferably 5 micrometers (5 x 10^"3 mm) or less, and even more preferably 1 micrometer (1 x 10^"3 mm) or less. The crystalline particles may be in the shape of equiaxed grains, rods, or platelets.

Examples of preferred ceramic-metal combinations for use in forming multiphase ceramic-metal composite articles comprises: B₄C/AI, SiC/AI, AIN/AI, TiB AI, AI.OJAI, SiB /Al, Si₃N₄/AI, SiC/Mg, SiC/Ti, SiC/Mg-AI, SiBx/Ti, B₄C/Ni, B₄C/Ti, B₄C/Cu, AI.O^Mg, AI₂O Ti, TiN/AI, TiC/AI, ZrB AI, ZrC/AI, AIB₁₂/AI, AIB AI, AIB₂₄C₄/AI, AIB₁₂/Ti, AIB₂₄C/Ti, TiN/Ti, TiC/ i, ZrO Ti,

TiC/Mo/Ni, SiC/Mo, TiB TiC/AI, TiB TiC/Ti, WC/Co, and WC/Co/Ni. The use of the subscript "x" represents that the compound can have varying stoichiometry. More preferred ceramic-metal combinations comprise: B₄C/AI, SiC/AI, SiB_g AI, TiB AI and SiC/Mg. Most preferably, the materials forming the complex-shaped ceramic-metal composite article of the present invention are chemically reactive systems such as aluminum-boron-carbide. In these chemically reactive systems, the metal component, after infiltration, can be depleted to form ceramic phases that modify article properties such as hardness. The aluminum-boron- carbide composite material includes at least one boron-carbide-containing phase and at least one aluminum-containing phase. Additionally, the phases may be admixed with a filler ceramic. The filler provides material for the finished article that does not adversely affect the desired properties of the ceramic-metal composite article. The filler can be selected from the group consisting of borides, carbides, nitrides, oxides, suicides, and mixtures and combinations thereof. The filler ceramic is preferably employed in an amount from 1 to 50 volume percent based on the volume of the multi-phase ceramic-based material.

The aluminum-boron-carbide composite article preferably includes the phases of B₄C, AIB₂₄C₄, AI^BC, AIB₂, AIB₁₂, AIB₁₂C₂, AI₄B_{1 3}C₄ and free metal Al. The most preferred material is a multi-phase material made of B₄C, Al, and at least three other ceramic phases, preferably, AIB₂₄C₄, Al_gBC, AI₄BC, and AIB₂. The B₄C grains are preferably surrounded by aluminum boride and aluminum boron carbide. In other words, the composite article has a continuous ceramic network of aluminum boron, boron carbide, and aluminum-boron- carbide.

The preparation of the coated ceramic-metal composite article involves initially forming a ceramic-metal composite article upon which a first smoothing layer will be formed. The ceramic-metal composite article is prepared by forming a ceramic substrate in the desired finished article shape and then infiltrating the ceramic substrate with a chosen metal. The selected ceramic is formed into the near net finished article shape. Any ceramic- forming process or processes may be used which allows the formation of shaped parts at or near net size and shape. Such ceramic-forming processes are well known in the art, for example, injection molding, slip casting, tape casting or chemical vapor deposition. Preferred ceramic-forming processes include injection molding or tape casting.

The next step in the process of forming a ceramic-metal article for use in the process of the invention involves infiltrating the shaped ceramic body with the chosen metal such that a shaped ceramic-metal composite article is formed. Infiltration is the process by which a metal, upon melting, forms a solid-liquid interface with a ceramic, with the metal as the liquid and the ceramic as the solid, and the metal moves into the pores of the ceramic material by capillary action. This process preferably forms a uniformly dispersed and fully dense ceramic-metal composite material. Infiltration can be performed by any method that is known in the industry, for example, U.S. Patents 4,702,770 and 4,834,938, both incorporated herein by reference. There are many well-known ways of infiltrating a metal into a ceramic body. Preferred methods of infiltration are heat infiltration, vacuum infiltration, pressure infiltration, and gravity/heat infiltration. When the infiltration is performed, the metal wets and permeates the pores of the ceramic that is in contact with the shaped metal body. The degree of wetting measured by the contact angle between the metal and the ceramic may be controlled by selecting temperature and time of infiltration. The temperature of infiltration is dependent upon the chosen metal. Infiltration is preferably performed at a temperature such that the metal is molten but below the temperature at which the metal rapidly evaporates. The preferred temperature for infiltration of the selected metal into the selected ceramic depends on the melting temperature of the selected metal. For aluminum, the preferred temperature for infiltration of the selected metal into the selected ceramic is 1200°C or less and more preferably from 1100°C or less. For example, the preferred temperature for infiltration of aluminum into a ceramic is from 750°C or greater, and more preferably 900°C or greater.

For each metal, exact temperature and time of infiltration can be established by contact angle measurements to determine when wetting conditions are achieved. Infiltration time is dependent on several factors, such as packing density, pore radius, void ratio, contact angle, viscosity, surface tension and sample size. Infiltration is preferably performed until the metal-infiltrated ceramic material is substantially dense. Preferably, the infiltration time for a metal selected from the preferred class of metals and a ceramic selected from the preferred class of ceramics is 0.1 hour or greater, more preferably 0.5 hour or greater, and even more preferably 1 hour or greater. Preferably, the infiltration time for a metal selected from the preferred class of metals and a ceramic selected from the preferred class of ceramics is 24 hours or less, more preferably 12 hours or less, and even more preferably 6 hours or less. For example, the preferred time for infiltration of aluminum into a 1 mm thick layer of boron carbide at 1100°C is 10 minutes. Infiltration can be accomplished at atmospheric pressure, subatmospheric pressures or superatmospheric pressures. The infiltration is preferably performed in an inert gas, such as argon or nitrogen. At superatmospheric pressure, the infiltration temperature can be lowered. Infiltration is preferably performed until the ceramic-metal composite article is densified to greater than 98 percent theoretical density, more preferably to greater than 99.5 percent theoretical density. Upon completion of the infiltration step, a fully infiltrated, complex-shaped ceramic- metal composite article is formed.

After infiltration, heat-treatment may be optionally performed on the ceramic- metal composite article in order to further tailor mechanical properties of the article. A preferred method of altering the microstructure of already infiltrated ceramic-metal composites involves post-infiltration heat-treatments of the previously infiltrated composites. The mechanical properties that can be tailored include fracture toughness, fracture strength, and hardness. This additional step of heating the ceramic-metal composite article at a selected temperature for a selected amount of time will decrease the amount of residual free metal and improve the uniformity of the multi-phase ceramic-based material. As a result of the post-infiltration heat-treatment, a slow growth of ceramic phases takes place. It is during this heat-treatment that the greatest control over the formation of multi-phases and the above-stated mechanical properties in the ceramic-metal composite article is achieved. The temperature at which the heat-treatment is performed is a temperature at which the residual free metal will decrease. Furthermore, the temperature at which the heat-treatment is performed is the lowest temperature at which chemical reactions in the solid state are taking place. A preferred method of altering the microstructure of already infiltrated ceramic-metal composites involves post-heat-treatments of already infiltrated composites at 650°C or greater, more preferably 700°C or greater. The maximum temperature for post-heat- treatment is the melting point of the metal in the ceramic-metal composite article. The time of heat-treatment is preferably long enough that the desired properties in the ceramic-metal composite article are achieved by altering the microstructure. For example, in the case of aluminum-boron-carbide, this additional step of heat-treating is preferably accomplished by heating the infiltrated body to a temperature of 660°C or greater, more preferably 700°C or greater, and even more preferably 800°C or greater. Preferably, the heat-treatment is accomplished at a temperature of 1500°C or less, more preferably at 1200°C or less, and even more preferably 1000°C or less. The preferable time period for the heat-treatment of aluminum-boron-carbide is from 1 hour or greater, more preferably 25 hours or greater. The heat-treatment may be performed in air or an inert atmosphere such as nitrogen or argon. Preferably, the heat-treatment is performed in air.

After infiltration and optional heat-treatment, the infiltrated body is cooled.

Optionally, the infiltrated body may be machined and polished into a final desired shape. It may be desirable to polish the infiltrated article, depending upon the end usage for the infiltrated article. For example, if the desired article is a computer hard disk, the surface of the disk should be polished to a substantially uniform average roughness value of between 3 and 2000 A.

The ceramic-metal composite article is then coated with a smoothing material under conditions such that a first smoothing layer is formed on one or more face(s) of the ceramic-metal composite article with few or no surface defects in the first smoothing layer. The contacting of the smoothing material with the ceramic-metal composite article can be performed in any order in any combination with any of the following pretreatment steps. Once the smoothing material has been contacted with the ceramic-metal composite article, the smoothing material forms a layer on the surface of and within the defects in the surface of the ceramic-metal substrate due to the electrochemical forces present between the smoothing material and the metallic component of the ceramic-metal composite. The pretreatment steps can include any combination of a cleaning step, an etch step, a desmutting step and a zincating step. The pretreatment steps are performed using various bath sizes of the materials called for in each pretreatment step. The bath size varies depending upon the size of the article desired to be pretreated. The invention can be performed while eliminating one or more of the pretreatment steps or by combining or repeating one or more of the steps. Furthermore, any or all of the pretreatment steps can be preceded or followed by a wash with deionized water in order to prevent cross-contamination between the pretreatment steps. The wash can be performed by any method known to one skilled in the art. For example, the wash can be performed by dipping the article in de- ionized water at room temperature. The wash time is preferably 1 second or greater, more preferably 2 seconds or greater and even more preferably 5 seconds or greater. The wash time is preferably 600 seconds or less, more preferably 300 seconds or less and even more preferably 120 seconds or less.

5 The cleaning step can be performed by any suitable method. The cleaning step can be performed by any method which removes organic soils and films formed during the manufacturing of the ceramic-metal composite substrate. For example, the soaking step can be performed by soaking the ceramic-metal composite article in a cleaner at an elevated temperature for a set period of time. Preferred cleaning agents are any aqueous alkaline 0 material. The time of soaking should be sufficient to remove the organic soils and films on the surface of the substrate. Examples of preferred soaking agents include Enbond™ NE- 5979, Niklad Alprep™(NS) 2-4 or 230, Fidelity™ 3152 or AD-68F™. Preferably, the ceramic- metal composite article is soaked for 0.1 minute or greater, more preferably 0.5 minutes or greater, and even more preferably 1 minute or greater. Preferably, the ceramic-metal 5 composite article is soaked for 60 minutes or less, more preferably 30 minutes or less, and even more preferably 15 minutes or less. Preferably, the soaking is performed at a temperature of 50°F (10°C) or greater, more preferably 60°F (15.6°C) or greater and even more preferably 65°F (18.3°C) or greater. Preferably, the soaking is performed at a temperature of 200°F (93.3°C) or less, more preferably 180°F (82.2°C) or less, and even o more preferably 170°F (76.7°C) or less.

The etch step can also be performed by any method known to one skilled in the art. The etch step dissolves and disperses a variety of soils from the uncoated ceramic- metal composite substrate. Etching removes any unwanted material that may be embedded in the surface of the uncoated ceramic-metal composite substrate. Etching also may remove 5 certain surface oxides and increase the surface area to promote adhesion of the final coatings. The etch step, for example, can be performed in either an alkaline or acidic solution at an elevated temperature for a set period of time. Preferably, the etch step is an acid etch step. Preferred materials for the etch step include phosphoric acid, Actane™ E-10, Alprep™ 230,245, Fidelity™ 3133 or AD-101 F™. The time of the etch should be sufficient to o remove the variety of soils remaining on the surface of the substrate. Preferably, the ceramic-metal composite article is etched for 0.1 minute or greater, more preferably 0.2 minutes or greater and even more preferably 0.5 minutes or greater. Preferably, the ceramic-metal composite article is etched for 30 minutes or less, more preferably 15 minutes or less and even more preferably 5 minutes or less. Preferably, the acid etch is performed at a temperature of 100°F (37.8°C) or greater, more preferably 140°F (60°C) or greater and even more preferably 150°F (65.6°C) or greater. Preferably, the acid etch is performed at a temperature of 200°F (93.3°C) or less, more preferably 170°F (76.7°C) or less and even more preferably 165°F (73.9°C) or less.

The desmutting step can be performed by any method known to one skilled in the art. The etch step can produce a smut or residue on the surface of the ceramic-metal composite substrate. The nature of the smut depends on the type of alloy used for infiltration. The desmutting step is used to remove these residues. The desmutting agent can be any agent which will remove the smut from the surface of the ceramic-metal composite. Preferred desmutting agents include 50 percent HNO₃ and Alprep™ 290. Preferably, the desmutting agent used is a 50 percent nitric acid. For example, the step can be performed using 50 percent HNO₃ desmut at room temperature for 30 seconds. The time of the desmutting step should be sufficient to remove the smut from the surface of the substrate. Preferably, the ceramic-metal composite article is desmutted for 0.1 minute or greater, more preferably 0.5 minutes or greater, and even more preferably 1 minute or greater. Preferably, the ceramic-metal composite article is desmutted for 30 minutes or less, more preferably 15 minutes or less, and even more preferably 5 minutes or less. Preferably, the desmutting step is performed at a temperature of 50°F (10°C) or greater, more preferably 60°F (15.6°C) or greater, and even more preferably 70°F (21.1 °C) or greater. Preferably, the desmutting step is performed at a temperature of 150°F (65.6°C) or less, more preferably 130°F (54.4°C) or less, and even more preferably 110°F (43.3°C) or less.

The zincating step can also be performed by any method known to one skilled in the art. Preferably, one or more alkaline zincating steps are used. The zincating step is performed because of the relatively impervious and rapidly forming oxide film which forms on the ceramic-metal composite substrate. The oxide layer produces poor adhesion of any later coatings layers on the ceramic-metal substrate. Zincating replaces the oxide film with a thin layer of zinc and thus promotes adhesion of the subsequent plating layer. The time of zincating should be sufficient to lay down a layer of zinc on the surface of the ceramic-metal composite substrate. Preferably, the ceramic-metal composite article is zincated for

1 second or greater, more preferably 5 seconds or greater, and even more preferably 15 seconds or greater. Preferably, the ceramic-metal composite article is zincated for

600 seconds or less, more preferably 300 seconds or less, and even more preferably 120 seconds or less. Preferably, the zincating is performed at a temperature of 50°F (10 °C) or greater, more preferably 60°F (15.6°C) or greater and even more preferably 65°F (18.3°C) or greater. Preferably, the zincating is performed at a temperature of 100°F (37.8°C) or less, more preferably 85°F (29.4°C) or less and even more preferably 75°F (23.9°C) or less.

The step of forming a layer of the smoothing material on one or more face(s) of the shaped ceramic-metal composite article by contacting the smoothing material with the shaped ceramic-composite article can be performed before, during or after any of the above described pretreatment steps. Preferably, the contacting of the smoothing material is performed after any combination of the above pretreatment steps. The smoothing materials useful in this invention for contacting with the shaped ceramic-metal composite article to form a first smoothing layer(s) are chosen based on their ability to allow lateral and vertical growth of any subsequent coating deposited on and within the ceramic-metal composite article. The smoothing layer allows the subsequent plating or coating layer to fill the defects on and within the surface of the ceramic-metal substrate. Preferred smoothing materials are selected from Groups VIIB, VIII or IB under the CAS notation and combinations thereof. More preferred smoothing materials are copper, nickel, silver and gold and combinations thereof. A most preferred smoothing material is copper. Preferably, the chosen smoothing material is in the form of a salt of the chosen smoothing material. The smoothing material can be a liquid, powder or combination thereof.

The chosen smoothing material is contacted with the shaped ceramic-metal composite article in order to form a first smoothing layer(s) of the smoothing material on one or more face(s) of the shaped ceramic-metal composite article. The smoothing material can be contacted with the shaped ceramic-metal composite article by any means which results in the formation of a first smoothing layer(s) of the smoothing material on one or more face(s) of the shaped ceramic-metal composite article such as atomized liquid spraying, dipping, spinning, brushing, rolling, padding, screening (for example screen printing), soluble gel coating, electrostatic spraying, electrophoretic depositing, casting (for example tape casting) and combinations thereof. See, for example. Principles of Ceramic Processing. James Reed, 1988, or Handbook of Tribology. Materials. Coatings, and Surface Treatments, supra, and Deposition Techniques for Films and Coatings. R.F. Bunshah et al., Neyes Publications, New Jersey, 1982, relevant parts of each incorporated herein by reference. The first smoothing layer can be a continuous layer, discontinuous layer or a layer can be deposited in a pattern on the ceramic body. Patterns may be formed by a screen printing or a masking technique. More than one smoothing layer of the smoothing material and more than one smoothing material can be contacted with the ceramic-metal composite article.

Preferably, the smoothing material is blended with other materials into a mixture in order to facilitate the contacting of the smoothing material with the surface of the shaped ceramic-metal composite article. This can be accomplished by any method known to one skilled in the art. See, for example, Metal Finishing Guidebook and Directory. 1995, Vol. 93, Number 1A. The smoothing material or its salt can be mixed with a liquid solvent and one or more of the following materials to form a mixture: a complexing agent, a stabilizer and a base. The liquid solvent can be any solvent which dissolves the smoothing material or ionizes its salt to form a mixture. Examples of useful solvents are water, acetonitrile and alcohols. A preferred solvent is water. The complexing agent can be any agent which slows down the deposition of the smoothing material. The complexing agent can be any ligan containing materials. Examples of useful complexing agents are NaKC₄H₄O₆4H₂O, ethylenediamine or combinations thereof. The stabilizer can be any stabilizer which buffers and stabilizes the mixture. Examples of useful stabilizers include carbonates. A preferred stabilizer is Na₂CO₃. The base can be any base which dissolves the contaminants on the surface of the ceramic-metal substrates. Examples of useful bases include NaOH, KOH or combinations thereof.

The amount of smoothing material used to form the smoothing layer is any amount which will provide a layer on the surface of and within the surface defects of the ceramic-metal substrate. Preferably, the amount of smoothing material used in the smoothing layer mixture is 15 g L or greater, more preferably 10 g/L or greater, and even more preferably 5 g/L or greater, based on the amount of liquid solvent. Preferably, the amount of smoothing material used in the smoothing layer is 100 g/L or less, more preferably 50 g/L or less, and even more preferably 30 g/L or less, based on the amount of liquid solvent.

The amount of the one or more materials such as a complexing agent, stabilizer and base used in the smoothing layer mixture is the amount sufficient to form a smoothing layer on the surface of the ceramic-metal substrate. Preferably, the amount of complexing agent is 1 g/L or greater, more preferably 10 g/L or greater, and even more preferably 30 g/L or greater, based on the amount of liquid solvent. Preferably, the amount of complexing agent is 100 g/L or less, more preferably 90 g/L or less, and even more preferably 80 g/L or less, based on the amount of liquid solvent. Preferably, the amount of stabilizer is 1 g/L or greater, more preferably 10 g L or greater, and even more preferably 30 g/L or greater, based on the amount of liquid solvent. Preferably, the amount of stabilizer is 100 g/L or less, more preferably 90 g/L or less, and even more preferably 80 g/L or less, based on the amount of liquid solvent. Preferably, the amount of base is 1 g/L or greater, more preferably 10 g/L or greater, and even more preferably 30 g/L or greater, based on the amount of liquid solvent. Preferably, the amount of base is 100 g/L or less, more preferably 90 g/L or less, and even more preferably 80 g/L or less, based on the amount of liquid solvent.

The time of contact of the smoothing mixture with the complex-shaped ceramic-metal composite article should be sufficient to layer the smoothing material in the desired amount on the article surface. Preferably, the time of contact is 1 second or greater, more preferably 5 seconds or greater, and even more preferably 10 seconds or greater. Preferably, the time of contact is 120 seconds or less, more preferably 60 seconds or less, and even more preferably 30 seconds or less.

The first smoothing layer(s) thickness generally is any thickness which is sufficient to result in a uniform layer on the surface of the complex-shaped ceramic-metal composite article. The first smoothing layer(s) thickness is dependent on the amount of smoothing material and layer porosity. The preferred first smoothing layer(s) thickness is 1 A or greater, more preferably 3 A or greater, and even more preferably 5 A or greater. The preferred first smoothing layer(s) thickness is 100 A or less, more preferred 50 A or less, and even more preferred 20 A or less. Alternatively, the smoothing layer need not be continuous as long as it is uniform. For example, the smoothing layer can be formed on the ceramic- metal substrate and machined off, leaving the remaining metal in a layer in the pores of the substrate but not in a continuous manner on the surface of the substrate.

Finally, a plating material is contacted with the smoothing-layered article in order to fill the defects on the surface of the composite article. The plating material is contacted with the smoothing-layered article to create a plating layer on one or more faces of the smoothing layered article. A preferred plating material, for example, is a nickel- phosphorus coating, however, other types of coatings can be used such as, for example, other metals and polymers. Preferably, the plating material useful in the invention is nickel- phosphorus. The nickel-phosphorus material is in a solution such as Enplate™ ADP-300, Niklad™719,1000, Fidelity™ 4355 or NIMUDEN HDX™. The plating method may be any that provides dense coating, such as atomic deposition, particulate deposition, bulk coating, or surface modification. Preferably, the methods used to coat the article are electroplating or electroless plating. More preferably, the method of coating is eiectroless plating. For example, an electroless nickel plating solution may be used for a variable amount of time. Preferably, the time of contact is 1 minute or greater, more preferably 5 minutes or greater, and even more preferably 10 minutes or greater. Preferably, the time of contact is 120 minutes or less, more preferably 100 minutes or less and even more preferably 90 minutes or less. Preferably, the temperature at which the contacting of the plating material with the smoothing-layered article occurs is from 100^°F (37.8°C) or greater, more preferably 150°F (65.6°C) or greater, and even more preferably 160°F (71.1°C) or greater. Preferably, the temperature at which the contacting of the plating material with the smoothing layered article occurs is from 212° F (100°C) or less, more preferably 200°F (93.3°C) or less, and even more preferably 195°F (90.6°C) or less.

The coating itself may be further treated to provide a textured surface either over the entire surface or a portion of the surface. Texture is useful on the surface of the coated ceramic-metal substrate in order for it to be utilized in many applications such as computer hard disks where a smooth surface presents a stiction problem when the head is parked. In the application of computer hard disks, if the surface does not have any texture, it would be difficult for the reading head to stop and start on the mirror smooth surface. The further treatment may be accomplished by techniques such as mechanical techniques, chemical or optical techniques, electrical techniques, or a combination thereof. The smoothness of the surface of the coated ceramic-metal composite article is sufficient to allow its use in the previously stated applications. Preferably, the smoothness of the surface of the coated article is 5 A or greater, more preferably 7 A or greater, and even more preferably 9 A or greater of Ra as measured on a WYKO RST Plus Interferometer. Preferably, the smoothness of the surface of the coated article is 30 A or less and more preferably 25 A or less, and even more preferably 15 A or less of Ra as measured on a WYKO RST Plus Interferometer.

The process for preparing the coated complex-shaped ceramic-metal composite articles allows the creation of coated complex-shaped ceramic-metal composite articles with few or no surface defects after coating. Preferred products of this invention are computer hard disks and hard disk drive components, wherein the material has a high hardness, a high wear resistance, a high fracture toughness, a high damping capability, a low density, and a high specific stiffness and is electrically conductive. Examples of computer hard disk drive components are hard disks, E-blocks, actuators, sliders, load beams, support arms, actuator bearings, spacers, clamps, spindles, ball bearings, thrust bearings, journal bearings, base plates, housings or covers. There are also many other applications for complex-shaped ceramic metal composite articles such as pressure housings, automotive engine parts, brake systems or any part that requires infiltration.

The following are included for illustrative purposes only and are not intended to limit the scope of the claims.

Example 1

Two clean aluminum-boron-carbide computer disk substrates having similar visual characteristics and from the same manufacturing batch were pretreated. The pretreatment consists of precleaning with ethanol and water followed by soaking at 65°C for 10 minutes in 3152 Soak Cleaner, soaking at 65°C for 2 minutes in 3133 Acid Etch, a 50 percent HNO₃ desmut at room temperature for 1 minutes and zincating in Zincate 3116 at room temperature for 30 seconds. Each of the above pretreatment steps was followed by a thorough wash with de-ionized water.

A copper immersion solution was used for 15 seconds on one of the pretreated aluminum-boron-carbide disk. The copper immersion solution consisted of 945.87g of NaKC₄H₄O₆4H₂O, 229.36g of NaOH, 208.58g of CuSO₄ 5H₂O and 137.37g of Na₂CO₃. All these ingredients were dissolved in 10.64 liters of 18 M de-ionized water.

While the one substrate was still wet from the copper immersion solution, both substrates were then placed in 4355 electroless nickel plating solution for a variable amount of time, washed and polished to a Ra of 20-30 Angstroms on a Strasbaugh production disk polisher using SpeedFam's Diskϋte 1312 polishing slurry. The nickel plated aluminum- boron-carbide disk substrate with a first smoothing layer and a nickel-phosphorus layer that was pretreated with the copper smoothing solution displayed significantly lower number of defects as compared to the control disk (substrate without copper smoothing layer). Example 2

An aluminum substrate was pretreated in the same manner as Example 1. A smoothing layer of copper was formed on the substrate. After the plating layer of nickel- phosphorus was formed on the smoothing-layered substrate, the aluminum disk displayed a significant amount of defects.

Claims

1. A process for preparing a coated composite article, comprising the sequential steps of:

(a) contacting a metal selected from the group consisting of copper, silver, gold, salts thereof and mixtures thereof with at least one face of a ceramic-metal composite article to form a smoothing layer of metal and

(b) forming a plating layer on said face.

2. The process of Claim 1 wherein the ceramic of the shaped ceramic-metal composite body is a boride, oxide, carbide, nitride, suicide or combinations thereof.

3. The process of Claim 2 wherein the metal of the shaped ceramic-metal composite is silicon, magnesium, aluminum, titanium, vanadium, chromium, iron, copper, nickel, cobalt, tantalum, tungsten, molybdenum, zirconium, niobium or combinations thereof.

4. The process of Claim 3 wherein the metal is aluminum and the ceramic is boron-carbide.

5. The process of Claim 4 wherein the smoothing layer(s) thickness is from 1 to 100 A.

6. The process of Claim 5 wherein the plating layer is nickel-phosphorus.

7. The process of Claim 1 wherein the coated ceramic-metal composite article has no defects of more than 100 micrometers on the surface of the coated article.

8 A coated composite article obtainable by the process of any one of the preceding claims.

9. A coated ceramic-metal composite article comprised of a shaped ceramic- metal composite article having a first smoothing layer comprised of gold, silver, copper or combination thereof and a plating layer on said smoothing layer on at least one face of the shaped ceramic-metal composite, wherein the coated ceramic-metal composite article has no defects of more than 100 micrometers on the surface of said coated face.

10. The coated ceramic-metal composite article of Claim 9 wherein the shaped ceramic-metal composite is an aluminum-boron-carbide composite.

11. The coated ceramic-metal article of Claim 10 wherein the plating layer is nickel-phosphorous.

12. The coated ceramic-metal article of Claim 9 wherein the article is a computer hard disk, E-block, actuator, slider, load beam, support arm, actuator bearing, spacer, clamp, spindle, ball bearing, thrust bearing, journal bearing, base plate, housing or cover.