DE4110005A1 - Composite body used for cutting tools - coated with fine crystalline alpha-aluminium oxide to improve adhesion, wear and corrosion resistance - Google Patents

Abstract

Composite body made of substrate of steel or Ni-W Co-based alloy with one or more protective layer, one of which is alpha - Al2O3, in which the alpha - Al2O3 is produced in a fine crystalline form confined by measuring the half value of an interference line produced by irradiating the coating with Cu-K (alpha) x-ray. The half value width is at least three times the width of a powder formed body. USE/ADVANTAGE - In coatings for cutting tools, high temp. material esp. for firing chambers or as wear resistance material for parts which rub together such as cam shafts. Coating has improved adhesion, wear and corrosion resistance properties, esp. at high temps

Description

The invention relates to a composite body consisting of a substrate body made of steel or nickel or cobalt base alloys and one or more surface protective layers, at least one of which consists of α-Al ₂ O ₃ , further the use of this composite body and a process for its manufacture .

Already in DE 24 28 530 A1, a method for protecting a metal part against corrosion and wear is proposed, which in the pure or alloyed state contains at least one element belonging to Group _{IB of} the periodic system of the elements and in which the surface thereof Partly a layer of amorphous and transparent aluminum oxide is applied by chemical vapor deposition. However, the amorphous layers applied at temperatures between 300 and 800 ° C. are less stable against thermal effects than, for example, the modification of aluminum oxide (α-Al ₂ O ₃ ) known as corundum.

Wear-reducing protective layers made of aluminum oxide are fer ner known for example from DE 22 33 700 C2, where before is struck, a hard metal substrate body made of a Mixture of at least one metal serving as a binder and at least one metal carbide of high hardness, with a Be made of aluminum oxide or zirconium oxide layering.

Basically, it is also known to be aluminum oxide layers Use protective layers against hot gas corrosion, e.g. B. in Internal combustion engines. In this case, next to me mechanical stability of the coating also has special requirements the tightness of the layer. According to the state  technology can only do this by comparative (approx. 500 µm) thick ceramic layers made by thermal spraying can be achieved.

As in particular in DE 28 25 009 C2, column 2, line len 28 ff., are hard, polycrystalline and compact α-alumina layers normally only with Abla production temperatures above 950 ° C. At low rigorous deposition temperatures are according to the state of the art nik loose and powdery deposits obtained from the γ modification and / or the ϑ modification of the aluminum oxide consist. At deposition temperatures of around 1000 ° C and above however, the alumina phase is usually the one for the coating of tools considered suitable -Modification. To face the danger, multi-phase alumi nium oxide coatings, which are said to be at low separation temperatures occur than 1000 ° C and the one considerable mechanical weakness and therefore the reason for before are early tool defects, it is proposed that the aluminum minium oxide coating entirely or at least 85% from the -Modification exists and that a possibly from the α-Modification existing residue on the surface areas or spots with a size of at most 10 microns. To Ab the CVD process at temperatures of approx. 1000 ° C suggested. However, this coating tends to Exposure to high temperatures to convert to α modification, so that cracks arise in the layer, which in particular when exposed to hot gas corrosion failure.

It is therefore an object of the present invention to begin with mentioned composite body with regard to the protective effect and mechanical wear properties of α-alumina Improve layer.  

This object is achieved by the composite body according to claim 1 ge, the characteristic feature of which consists in a finely crystalline structure of the α-aluminum oxide layer. The X-ray diffraction diagram provides a measure of the crystal size of the polycrystalline α-aluminum oxide structure. The line widths of the interference lines are narrower with the CuK _α radiation used and with the same angular position 2R of the counter tube registering the radiation and with the same aperture of the radiation collimator, the larger the interfering single crystal crystallites or the larger the mean grain size of the po lycrystalline material (here α-aluminum oxide). This relationship is described by the formula B _1/2 = k · λ / (<d <· cos R) derived by W. Scherrer. λ is the wavelength of the X-rays, <d <the mean linear extent of the reflecting crystallite, R the glancing angle and k a constant. The line width of the X-ray diffraction diagrams can thus be used as an easily accessible measure of the average grain size, particularly in the case of very fine, submicroscopic crystals. The half-width of the diffraction line measured with the same X-radiation when examining a powdery body made of α-Al ₂ O ₃ or an α-Al ₂ O ₃ layer applied with a CVD method at 1000 ° C. to 1100 ° C. is used as a comparison variable.

The X-ray diffraction line indicated by the Miller indices ( 113 ) and occurring at the diffraction angle of 43.4 ° of the 2 R scale for CuK _α -radiation radiation is preferably used as a measure, the half width, preferably at least four times the half width of the corresponding X-ray diffraction line of powder Alumina as a measure of the fine grain of the hexagonally packed crystal structure of the aluminum oxide.

According to a development of the invention, the thickness of the aluminum minium oxide coating 1 to 20 µm, preferably 2 to 10 µm, be.  

According to a further embodiment of the invention, there is the Be Layering made of aluminum oxide from 49 to 52.5% by mass of aluminum nium, 46 to 47.5 mass% oxygen and 0.5 to 3.5 mass% Chlorine.

Depending on the application, there are also protective layers that lasts with other layers of carbides, carbonitrides, Nitrides, borides and / or oxides of the elements of groups IVa to VIa of the periodic table were combined. Hereby expressly multilayer coatings made of aluminum oxide and Ti tancarbide and titanium nitride.

The protective layers of fine-crystalline α-Al ₂ O ₃ designed according to the main claim have the state of the art on suitable properties according to the results of the tests carried out. However, protective layers, the coating of which consisted only partly of ultra-crystalline α-Al ₂ O ₃ , but also partly of amorphous Al ₂ O ₃ or Al ₂ O ₃ with nanocrystalline structural components, showed significantly improved Ge compared to the prior art performance characteristics. Nanocrystalline structure components have crystallites of only a few nanometers in size and, due to their small size, cannot generate well-defined X-ray interferences. In a further embodiment of the invention it is therefore provided to use coatings which partly consist of very fine crystalline α-Al ₂ O ₃ and / or nanocrystalline Al ₂ O ₃ and partly consist of amorphous Al ₂ O ₃ . Protective layers which consisted entirely of amorphous Al ₂ O ₃ , however, proved to be unusable, since these coatings were very brittle and tended to crack and split under thermal and mechanical loads. In an amorphous material there is no long-range order or periodicity in the spatial arrangement of the atoms. Evidence as to whether a coating is ultra-crystalline and / or nanocrystalline or completely amorphous can also be obtained by examining it with an electron microscope. In the case of ultra-fine crystallinity or nanocrystallinity, interference patterns (Debye-Scherrer rings) are observed, which can be clearly assigned to specific network levels of the relevant crystal phases. A further embodiment of the invention is based on this.

The protective layer according to the invention can have great advantages various technical purposes. An on application is based on tightness and oxidation durability of the protective layer. Since the through the fiction layers produced by the process have compressive stresses sen, the coating remains even at high temperatures and the resulting expansion of the base body is still tight, d. H. there are no cracks. The protective layer is therefore from excellent for lining combustion chambers or for protecting suitable for moving parts in combustion chambers.

Surprisingly, it was found that the inventive Protective layers have excellent adhesion to metal have chemical material, especially on steel material. There Alumina against steel as a counter body a very low Has coefficient of friction, the invention Protective layer also to reduce friction and wear reduction of parts rubbing against each other, such as. B. with cams waves, be applied.

The excellent adhesiveness can improve the Adhesion of other hard material layers can be exploited. So could through on a base made of tool steel worn intermediate layer from the layer according to the invention fabric and an upper hard material layer made of titanium nitride Adhesion of the hard material layer can be significantly improved, compared to a tool steel that only with titanium nitride was coated. Finally, the invention is suitable Protective layer also to reduce tool wear from tool steels.  

For the production of the fine-grained α-aluminum oxide coating According to the invention, a plasma CVD method is used for substrates temperatures of 400 to 750 ° C applied, in which the plasma acti vation on the substrate body connected as a cathode with egg ner pulsed DC voltage is brought about. The low ones Coating temperatures by selecting the plasma CVD process rens lead to an improvement in the adhesion of aluminum oxide surface layer. The substrate body is except for the Support points completely and evenly from the layer covers without the Ab otherwise occurring in PVD processes shading effects. However, the coating is preferred performed at temperatures between 450 and 650 ° C. The ge pulsed DC voltage has maximum values between 200 and 900 V.

This further improves the quality of the coating sert that between the positive DC voltage pulses (Rectangular pulses) a residual DC voltage in the pulse pauses that is larger than the lowest ionization potential of the gas molecules involved in the CVD process, each but not greater than 50% of the maximum value of the pulsed DC voltage is. It does not primarily come down to the chip curve or the uniformity of the residual DC voltage on, but only on the fact that between two rectangular pulses, the residual DC voltage is always greater than said ionization potential. The following are some of the Ios relevant to the alumina CVD process Potentials of specified:

H: 13.5 eV,
H₂: 15.8 eV,
Ar: 15.7 eV,
O: 13.6 eV,
O₂: 12.1 eV,
AlCl₃: 11.0 eV.

According to a development of the invention, the ratio of Residual DC voltage to the maximum value of the pulsed DC voltage between 0.02 and 0.5.  

The period of the pulsed DC voltage should be preferred wise lie between 20 µs and 20 ms, one under the Pe the duration of a rectangular pulse and a pulse pause understands. The ratio of the pulse duration is preferred selected for the period between 0.1 to 0.6. The Parame ter are finally set so that a layer of wax speed of 0.5 to 10 µm / h is reached.

The process described for aluminum oxide coating is in principle already described in DE 38 41 730 A1 and can also be used to coat various other hard substances such as carbides, nitrides, borides, silicides and oxides with a particularly high hardness and a high melting point, so z. B. titanium carbide, titanium nitride, titanium carbonitride, zirconia oxide, boron carbide, silicon carbide and titanium diboride used , but it was surprising that contrary to the in the Fears expressed in prior art literature the protective layers an unexpectedly fine-grained α-alumina modification without further crystalline phases exhibited.

Preferred is as a hard gas-forming reactive gas atmosphere made of aluminum chloride, carbon dioxide and hydrogen, which by a glow discharge is partially ionized. As preferred gas pressure during coating are 200 to 300 Pascals set.

Developments of the invention and compared to the state of the Advantages resulting from technology are explained below with the aid of Drawings explained. Show it

FIG. 1 shows the structure of coating a with the present invention fine-crystalline α-Al ₂ O ₃ coated Stahlkör pers; scanning electron micrograph, magnification 8000: 1,

Fig. 2, the line profile of a known prior art compact alumina sample,

Fig. 3 shows the line profile of the α-aluminum oxide protective layer according to the invention on a steel substrate.

The deposited α-aluminum oxide has a very fine-grained structure. This microstructure can be made visible through examinations with a scanning electron microscope. As shown in FIG. 1, the protective layer according to the invention has a very fine, pore-free and crack-free structure.

It should be sent to the explanation of FIGS. 2 and 3, the remark that each crystal structure, such as. B. the α-Al ₂ O ₃ or the α-Al ₂ O ₃ at certain diffraction angles 2R, which shows interference lines characterized by the so-called Miller indices. For example, structure-specific interference lines occur with γ-Al ₂ O ₃ at different diffraction angles 2R than with α-Al ₂ O ₃ , so that an X-ray diffraction pattern is seen as a kind of fingerprint for the identification of crystal structures and the modifications of a solid substance can be.

As already mentioned above, the half-widths are in the middle direct proportional relationship to the mean size of the reflective crystal.

Fig. 3 shows a section of the line profile of a compact aluminum body compact. The half-value line width of the (113) reflex is 0.2 ° of the 2R scale when using CuK _α radiation.

In contrast, the half-width of the α-Al ₂ O ₃ line of the (113) reflex of a sample produced according to the invention is 1.05 ° of the 2 R scale (see Figure 3). Composite bodies with the fine-crystalline α-Al ₂ O ₃ coating according to the invention thus have a line width which is at least three times, in the present case five times the natural line width arising in the case of compact bodies (see FIG. 3).

Tests have shown that the resistance of Me materials against hot gas corrosion through the application of protective layer according to the invention is improved considerably has been. So showed with the protective layer according to the invention Sheets made of heat-resistant steel 1.4712 in oxidie render atmosphere at 1200 ° C after aging for 24 hours no visible and measurable changes while uncoated Sheets already showed a strong scaling.

It was found that the protective layers also apply to so-called called superalloys (nickel-based alloys and cobaltba sis alloys) with excellent adhesion. After aging at 1200 ° C. for 24 hours, the layers showed an undiminished adhesive strength.

To investigate the friction properties of the protective layer two outer rings of a ball bearing made of steel 1.3505 (100Cr6) of 68 mm outside diameter ground on the face and polished and one of the two rings with an inventive Provide protective layer. Both rings were in one fixture mounted on a vertical axis so that the polished or coated side of the ring got a horizontal position. On the polished or polished and coated surface of the ring a fixed steel ball with a diameter of 4 mm was placed and loaded with 50 N. The rings were now at 300 revolutions rotated per minute. After 120 minutes of testing, the Friction mark with a measuring device for determining surfaces roughness examined. During the uncoated trial body showed a clear deepening in the rubbing track (approx. 2 up to 3 µm), could be in the rubbing traces of the coated body no abrasion can be found.  

The protective layer according to the invention was also suitable as an adhesion-improving intermediate layer in the coating of tool steels (high-speed steel 1.3343) with Titanni trid examined. It was found that the adhesive strength of a outer, about 5 µm thick titanium nitride layer by an approx. 1 to 2 microns thick intermediate layer of the invention fine-grained aluminum oxide can be significantly improved could. This was checked by an apparatus in which one Diamond cone tip with 0.1 mm radius of curvature also grow the bearing load was carried over the coated surface. During a state of the art applied Titanni trid layer at 400 N load caused chipping, such flaking with the intermediate according to the invention layer only observed at 900 N contact load.

Finally, the effect of the protective layer on users to improve the life of coated spirals drills examined. It is known that the lifespan of High-speed steel drills by applying a Ti tannitride coating can be significantly improved. Therefore served as comparison tools, drills of 6 mm diameter, which were coated with titanium nitride by PVD. In the attempt finished, but uncoated drills with egg ner about 2 microns thick protective layer according to the invention provided. Both with these and with the titanium nitride coated Drills were holes in a plate made of 1.7225 steel (42CrMo4) drilled. While coated with the titanium nitride Drill 250 holes until the bit becomes blunt the corresponding number of holes was included the drill 550 coated according to the invention.

Claims (20)

1. Composite body consisting of a substrate body made of steel or a nickel or cobalt-based alloy and egg ner or more surface protective layers, of which at least one consists of α-Al ₂ O ₃ , characterized in that the α-Al ₂ O ₃ layer has finely crystalline structure, whose half-widths of the diffraction lines measured with CuK _α- X-ray radiation have a half-width at least three times as large as that of a powdery compact body made of α-Al ₂ O ₃ or an α applied with a CVD method at 1000 to 1100 ° C -Al ₂ O ₃ layer. 2. Composite body according to claim 1, characterized in that the X-ray diffraction line indicated by the Miller indices (113) and occurring at the diffraction angle 43.4 ° of the 2R scale in the case of CuK _α X-ray radiation is at least three times, preferably at least four times the half-width of the corresponding one Has X-ray diffraction line of powdery α-alumina. 3. Composite body according to claim 1 or 2, characterized in that the thickness of the Al ₂ O ₃ coating is 1 to 12 µm, preferably 2 to 6 µm. 4. Composite body according to one of claims 1 to 3, characterized in that the coating of Al ₂ O ₃ from 49 to 52.5 mass% aluminum, 46 to 47.5 mass% oxygen and 0.5 to 3.5 Mass% chlorine exists. 5. Composite body according to one of claims 1 to 4, characterized in that the coating consists partly of the finest crystalline α-Al ₂ O ₃ and / or nanocrystalline Al ₂ O ₃ and partly of amorphous Al ₂ O ₃ . 6. Composite body according to one of claims 1 to 5, characterized in that the electron diffraction pattern of the coating made of aluminum oxide interference rings, the individual network planes of α-Al ₂ O ₃ can be assigned. 7. Composite body according to one of claims 1 to 6, characterized is characterized by one or more additional layers Carbides, carbonitrides, nitrides, borides and / or oxides the elements of groups IVa to VIa of the periodic table. 8. Composite body according to claim 7, characterized in that at least one layer consists of TiC and / or TiN. 9. Composite body according to claim 7 or 8, characterized records that the adjacent to the substrate body Layer an aluminum oxide layer according to claims 1 to 6. 10. Use of the composite body according to one of claims 1 to 9 as cutting material for machining, as High temperature material, preferably in combustion chambers, or as a wear-resistant material for rubbing against each other Parts, in particular camshafts. 11. A method for producing a surface coating of α-Al ₂ O ₃ on a steel or cast steel substrate body or a hard material layer deposited thereon, characterized in that the α-Al ₂ O ₃ coating at substrate temperatures of 400 to 750 ° C. is carried out by means of a plasma CVD method, the plasma activation being brought about on the substrate body connected as the cathode with a pulsed direct voltage. 12. The method according to claim 11, characterized in that the coating at a temperature between 450 and 650 ° C is carried out.   13. The method according to claim 11 or 12, characterized in net that the pulsed DC voltage maximum values between has 200 and 900 V. 14. The method according to any one of claims 11 to 13, characterized ge indicates that between the positive DC voltage pulses, preferably rectangular pulses, in the pulse pauses a residual DC voltage is maintained, the larger than the lowest ionization potential of the CVD Gas molecules involved, but not larger than process Is 50% of the maximum value of the pulsed DC voltage. 15. The method according to claim 14, characterized in that the ratio of the residual DC voltage to the maximum value of the pulsed DC voltage is between 0.2 and 0.5. 16. The method according to any one of claims 10 to 15, characterized ge indicates that the period of the pulsed equals voltage is between 20 µs and 20 ms. 17. The method according to any one of claims 10 to 16, characterized ge indicates that the ratio of the pulse length (pulse duration) to the period is between 0.1 to 0.6. 18. The method according to any one of claims 10 to 17, characterized ge indicates that the layer growth rate 0.5 is up to 10 µm / h. 19. The method according to any one of claims 10 to 18, characterized ge indicates that the hard material-forming reactive gas atmosphere sphere made of aluminum chloride, carbon dioxide and hydrogen exists and partially ionized by a glow discharge becomes. 20. The method according to any one of claims 10 to 19, characterized ge indicates that the gas pressure during coating between 100 and 400 Pa.