WO2013090987A1 - Procédé de traitement - Google Patents

Procédé de traitement Download PDF

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Publication number
WO2013090987A1
WO2013090987A1 PCT/AU2012/001544 AU2012001544W WO2013090987A1 WO 2013090987 A1 WO2013090987 A1 WO 2013090987A1 AU 2012001544 W AU2012001544 W AU 2012001544W WO 2013090987 A1 WO2013090987 A1 WO 2013090987A1
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WIPO (PCT)
Prior art keywords
metal
particles
nitride
metal alloy
substrate
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PCT/AU2012/001544
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English (en)
Inventor
Mingxing Zhang
Shoumou MIAO
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The University Of Queensland
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Priority claimed from AU2011905392A external-priority patent/AU2011905392A0/en
Application filed by The University Of Queensland filed Critical The University Of Queensland
Publication of WO2013090987A1 publication Critical patent/WO2013090987A1/fr

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02—Pretreatment of the material to be coated

Definitions

  • the present invention relates to the treatment of metal or metal alloys to increase their wear resistance.
  • the present invention relates to the production of metal nitrides on corresponding metal substrates.
  • Light metals and light metal alloys such as titanium and titanium alloys, are attractive materials because of their superior combination of good mechanical properties and excellent corrosion resistance.
  • their poor tribological properties tend to restrict their usage. Attempts have therefore been made to improve the wear resistance of these materials, for example by surface treatment.
  • nitriding A commonly used surface treatment technique to enhance wear resistance of certain metals is nitriding.
  • the primary aim of this technique is to produce a hard surface layer of metal nitride with this layer providing improved wear and corrosion resistance. Due to these effects, nitriding is of great industrial interest.
  • nitriding of titanium and its alloys Due to its high chemical reactivity at elevated temperature, nitriding of titanium and its alloys is relatively difficult to carry out using conventional gas nitriding methods developed for ferrous alloys and steels.
  • various alternative nitriding techniques have been applied to titanium and titanium alloys to reduce the temperature at which nitriding takes place including the use of high-energy beam sources.
  • high-energy beam sources available for titanium alloys, including plasma ' nitriding, laser nitriding and ion-beam nitriding.
  • plasma nitriding in particular has already reached an industrial application stage in the biomedical field. Nonetheless, the massive investment in equipment and on-going energy consumption, and dimensional limitations have restricted the industrial applicability of these techniques.
  • the nitrogen sources used in this technique include pure nitrogen, nitrogen/argon gas mixtures, nitrogen/hydrogen gas mixtures and ammonia.
  • pure nitrogen in order to avoid hydrogen embrittlement and reduce environmental pollution the use of pure nitrogen is preferred.
  • ultra pure nitrogen environment purity of at least 99.99%. This is very expensive and brings with it strict process control to avoid the purity of the nitrogen being compromised.
  • Lower purity grades of nitrogen, such as industrial grade nitrogen are cheaper but include oxygen at levels that present a problem with respect to oxidation of titanium/titanium alloy.
  • the present invention provides a method of forming a metal nitride or metal alloy nitride on a corresponding metal or a metal alloy substrate, which method comprises: covering the substrate with an oxygen-reacting composition comprising (a) , particles of a metal that oxidizes in preference to the metal or metal alloy substrate and that is unreactive with respect to the metal or metal alloy substrate and (b) particles of an inert material; and exposing the covered substrate to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce the metal nitride or metal alloy nitride on the substrate.
  • the present invention also provides a metal or metal alloy substrate that has been subjected to surface nitriding in accordance with the method of the present invention.
  • Figure 1 shows a schematic arrangement of a gas nitriding assembly that may be used to carry out the method of the present invention.
  • Figure 2 shows optical micrographs of cross-sections of pure Ti after nitriding at 950°C for 24 hours in (a) nitrogen flow, i.e. without oxygen-reacting composition and (b) using an oxygen-reacting composition of particulate Al 2 0 3 +5 wt.% particulate Mg.
  • Figure 3 shows SEM and optical images of cross-sections of pure Ti after gas nitriding in a powder mixture of A1 2 0 3 + 5 wi% Mg at temperatures from 650°C to 850 °C for 24 hours.
  • Figure 4 reports X-ray diffraction spectra of pure Ti samples after gas nitriding at 650°C and 800°C for 24 hours.
  • Figure 5 shows bright field TEM images of TiN/Ti 2 N and Ti 2 N/Ti interface and corresponding diffraction patterns of TiN, Ti 2 N and Ti on pure Ti after nitriding at 850°C for 24 hours in a powder mixture of Al 2 0 3 +5 wt.% Mg.
  • Figure 6 shows optical micrographs of cross-sections of pure Ti after gas nitriding in a powder mixture of Al 2 0 3 +5 wt.% Mg at 950°C for (a) 12 hours and (b) 24 hours.
  • Figure 7 shows optical micrographs on cross-sections on pure titanium after gas nitriding in a powder mixture of Al 2 O 3 +60 wt.% Cu at different temperature from 850°C to 1050°C for 24 h.
  • Figure 8 shows optical micrographs of the cross-section on pure titanium after gas nitriding in a powder mixture of Al 2 O 3 +60 wt.% Cu at 950 °C for (a) 12 hours and (b) 24 hours.
  • Figure 9 shows microhardness on surface and cross-sectional profiles of pure Ti after gas nitriding in a powder mixture of Al 2 0 3 +5 wt.% Mg at temperatures from 650°C to 850°C for 24 hours.
  • Figure 10 shows wear resistance, indicated by the wear depth and volume loss respectively, of the nitride layer on pure Ti and Ti-6A1-4V substrate measured at different loads for 30 minutes and compared to Ti-6A1-4V alloy.
  • Figure 11 shows SEM micrographs showing the morphology on worn surfaces nitrided at 850 °C for 24 h after wear testing at different loads for 30 min: (a) Ti6A14V, (b) TiN on pure Ti and (c) TiN on Ti6A14V.
  • Figure 12 shows average coefficient of friction (COF) of nitrided layer on pure Ti and Ti- 6A1-4V substrate measured at different loads for 30 minutes and compared toTi-6Al-4V alloy.
  • Figure 13 shows images of nitrided pure Ti and Ti-6A1-4V exposure to salt spray for different time compared to as-sprayed Ti, Ti-6A1-4V and 316SS.
  • Figure 14 shows average weight gain/loss of nitrided pure Ti and Ti-6A1-4V samples exposure to salt spray for different time compared to as-received pure Ti, Ti-6A1-4V and 316SS.
  • Figure 15 shows linear polarization behaviour of TiN on pure Ti and Ti6Al4V as compared with as-received 316SS, pure Ti and Ti-6A1-4V.
  • the invention involves on the formation of a metal nitride layer on the surface of metal or metal alloy substrates.
  • This nitride layer provides improved wear resistance.
  • the metal nitride layer forms by reactive growth when the surface of the substrate comes into contact and reacts with nitrogen at high temperature.
  • the present invention uses industrial grade nitrogen (also known as industry purity nitrogen) as the source of nitrogen for the nitriding reaction.
  • industrial grade nitrogen would normally present problems in this context since it contains an amount of oxygen sufficient to induce formation of undesired metal oxides on the substrate surface.
  • the presence of oxygen is mitigated by the use of an . oxygen-reacting composition with the result that oxidation of the substrate surface can be avoided.
  • the present invention may be used to coat the entire exterior surface of a substrate or only a part of the exterior surface. Surfaces not to be coated may be masked or otherwise prevented from being in contact with the composition that is used in the method.
  • the oxygen-reacting composition comprises a mixture of reactive metal particle and inert particles.
  • the reactive metal has two requirements in terms of reactivity. Firstly, the reactive metal must be one that, at the temperatures employed in the method of the invention, can more readily react with oxygen to form a metal oxide than the metal or metal alloy of the substrate. Considering that the measure of a material to oxidize (or lose electrons) is known as its oxidation potential, the reactive metal will have a higher oxidation potential than the metal or ' the metal alloy of the substrate.
  • the reactive metal particles do not react with the metal or metal alloy substrate under the operating conditions employed.
  • the reactive metal does not form any intermetallic species when contacting the substrate at elevated temperature.
  • the oxygen-reacting composition also comprises inert particles. These are particles of a material that are entirely unreactive during the method of the invention. Thus, the inert particles do not react with any components of the industrial grade nitrogen, with the metal or metal alloy substrate or the reactive metal particles.
  • the function of the inert particles is as a filler and to prevent sintering of the reactive metal particles under the elevated temperatures employed in the present invention.
  • the oxygen-reacting composition covers the substrate and the nitrogen-containing gas being used must pass/permeate through the composition before reaching the substrate surface.
  • An amount of reactive metal particles will also contact the surface of the substrate.
  • the reactive metal particles can play two roles in the method of the invention. On one hand. they are believed to react with oxygen in the industrial grade nitrogen thereby reducing the oxygen concentration. At the very least this will significantly limit the concentration of oxygen available for reaction at the substrate surface. At best no oxygen will be available for reaction at the substrate surface.
  • the reactive metal particles are believed to reduce metal oxide existing, or formed during the method of the invention, on the metal or metal alloy substrate. This would increase the surface area of metal or metal alloy available for reaction with nitrogen and/or mitigate any undesired oxidation of the substrate surface during application of the method of the invention.
  • the relative amounts and particles sizes of the reactive metal particles inert particles should be selected based on a number of practical considerations as follows:
  • the overall specific surface area of the reactive metal particles must be sufficient to achieve the necessary extent of reaction with oxygen present in the nitrogen source used. If too little reactive metal is present, the ability of the reactive metal to "mop up" oxygen may not be adequate.
  • the composition should be suitably permeable to the nitrogen source.
  • a low weight percentage mixture of reactive metal particles that are too small compared to the inert particles may be unfavourable, as voids between the various particles may be large enough to allow for oxygen to flow through the composition with minimum interaction with the reactive metal particles.
  • the characteristics of the composition in terms of weight proportions and particles sizes of components can be assessed and optimised by experiment.
  • the reactive metal particles and inert particles each have a mean particle size of about 5 ⁇ to about 100 ⁇ .
  • the weight percent of reactive metal particles can vary greatly, but is usually no more than about 65% by weight based on the total weight of the composition.
  • the present invention is believed to have particular utility in relation to titanium and titanium alloys substrates.
  • Various grades of pure titanium may be used, such as Grade 2.
  • Examples of titanium alloys that may be treated in accordance with the invention include Ti6Al4V, TilOV2Fe3Al, Ti6Cr5 o5V4Al, Til 100 and IMI834. This list is not exhaustive however and the usefulness of the present invention in relation to other grades of titanium and other titanium alloys may be assessed by experiment.
  • Mg particles suitable for use in the present invention would have a mean particle size of from about 5 ⁇ to about 100 ⁇ .
  • the use of atomized Mg powder with a mean particle size of 45 ⁇ may be preferred.
  • a typical composition of oxygen reactive powder can be obtained mixing Mg particles with inert particles so that the Mg particle concentration would be in the range of from about 2% to about 25% by weight with respect to the total weight of the composition.
  • Cu particles When the invention is applied in relation to titanium and titanium alloys, Cu particles would typically have a mean particle size of from about 1 ⁇ to about 50 ⁇ .
  • the use of Cu 101 Metal Flakes, 99.9% pure, with mean particle size of 1-5 ⁇ may be given by way of specific example.
  • a typical composition of oxygen reactive powder can be obtained mixing Cu particles with inert particles so that the Cu particle concentration would be in the range from about 2% to about 60% in weight with respect to the total weight of the oxygen-reacting composition.
  • Inert particles for the purposes of the present invention may be particles made of fused alumina (A1 2 0 3 ), SiC or Zr0 2 . Typically, the average particle size is from about 5 ⁇ to about 100 ⁇ , preferably of about 45 ⁇ .
  • the oxygen-reacting composition is prepared by simple mixing of the component particles. The homogeneity of the composition may be assessed to determine what constitutes adequate mixing.
  • the oxygen-reacting composition should be in intimate contact with the substrate surface and the composition may be loosely packed onto and around the substrate to facilitate this. It is preferred that there minimal or no voids/air gaps at the interface between the composition and substrate surface to ensure good contact between the substrate and the composition.
  • the composition should be provided as a relatively thick layer covering the surface of the substrate, for example at least about 5 mm thick, preferably at least about 10 mm thick, with the maximum thickness usually being about 20 mm.
  • the composition is packed around the substrate in a suitable container.
  • the composition should be used in dry form.
  • the relevant surface(s) of the substrate Prior to providing the composition on the substrate the relevant surface(s) of the substrate are preferably be cleaned (e.g. de-greased). It may also be desirable to prepare the surface(s) by machining or grinding as this may enhance the relevant reactions.
  • Figure 1 is a non-limiting representation of a possible set-up that can be used to carry out the method of present invention.
  • the bottom of a container is pre-filled with particles of an inert material (these may be the same or different material from the inert particles in the oxygen-reacting composition) in order to facilitate sample removal at the end of the nitriding treatment.
  • an inert material these may be the same or different material from the inert particles in the oxygen-reacting composition
  • a substrate is then position on top of the composition and the substrate then covered with oxygen-reacting composition sufficient to completely cover the substrate. At all stages of filling the composition can be loosely packed by gentle tamping.
  • the top of the container is then topped up with flakes or particles of the reactive metal used in the composition (possibly collected from machining of reactive metal ingots). These flakes/particles have been found to prevent oxidation of the oxygen- reacting composition during the subsequent heating stage.
  • the substrate and oxygen-reacting composition are exposed to an atmosphere of industrial grade nitrogen.
  • Industrial grade nitrogen is intended as any gas mixture containing approximately 99% by volume of nitrogen with the balance being oxygen and traces of inert gas.
  • the nitrogen mixture is produced by common industrial production techniques, i.e. cryogenic air distillation or membrane separation.
  • the industrial grade nitrogen contains about 99.5% of nitrogen by volume with the balance being oxygen and traces of inert gas.
  • the container and surrounding heating chamber are evacuated and backfilled with nitrogen.
  • the evacuation- backfill process can be repeated to minimize the presence of atmospheric air.
  • Industrial grade nitrogen is pumped into the top of the chamber via holes provided ' for that purpose and exits the container through other holes.
  • the container is continuously replenished with respect to nitrogen as reactant.
  • the container (and furnace) are evacuated and backfilled before heating takes place.
  • the prevailing temperature should be sufficiently high to allow nitrogen to chemically react with the metal or metal alloy substrate to form of metal-nitride or metal alloy nitride. This reaction is known per se and is believed to proceed in conventional manner.
  • the temperature will be between about 650°C and 1050°C. Heating is generally carried out for a number of hours, for example at least about 12 hours, such as up to 24 hours. It has been found that the nitride layer generally develops more rapidly at higher temperatures than at lower temperatures. However, the thickness of the nitride layer that is formed at one particular temperature can also be increased using a longer treatment time.
  • the nitride layer developed in accordance with the present invention should have a thickness of at least 5 ⁇ , for example from 5 ⁇ to 200 ⁇ .
  • the thickness depends upon the coating parameters used, primarily on the temperature and time of heating, and these may be manipulated and optimized as necessary. The optimum coating thickness for a given application may also be determined. After heating the substrate is allowed to cool before removal from excess coating composition. The coating on the substrate may then be subjected to surface finishing as may be required, for example to maintain dimensional tolerances. Grinding, machining and/or polishing may be applied to the coated substrate.
  • Coated substrates may be assessed for coating quality and analysed using a variety of techniques, such as optical microscopy, electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), micro hardness testing and wear resistance testing, when appropriate.
  • techniques such as optical microscopy, electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), micro hardness testing and wear resistance testing, when appropriate.
  • the invention may also be applied to provide wear resistant coatings on light metal or light metal alloys of machine components.
  • surface hardening of such components is undertaken by using gas nitriding, chemical/physical vapour deposition, thermal spraying or laser gas alloying in cases where components are subject to sliding forces or friction.
  • a method of providing a wear resistant coating on a surface of a light metal or light metal alloy machine component comprises forming a metal nitride or metal alloy nitride on the surface by covering the surface with an oxygen-reacting composition comprising (a) particles of a metal that oxidizes in preference to the light metal or light metal alloy and that is unreactive with respect to the light metal or light metal alloy and (b) particles of an inert material and exposing the covered surface to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce a metal nitride or metal alloy nitride on the surface.
  • the present invention may also find use in providing wear resistant coatings on a metal or metal alloy surface of a medical implants, such as joint replacement, where excellent biocompatibility combined with high wear resistance are required.
  • the method comprises forming a metal nitride or metal alloy nitride on the surface by covering the surface with an oxygen-reacting composition comprising (a) particles of a metal that oxidizes in preference to the metal or metal alloy of the surface and that is unreactive with respect to the metal or metal alloy of the surface and (b) particles of an inert material and exposing the covered surface to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce a metal nitride or metal alloy nitride on the surface.
  • application substrates include metal or metal alloy surfaces of gas turbine components, such as discs and blades, which can be nitrided to reduce fretting wear.
  • gas turbine components such as discs and blades
  • the invention may be applied to large scale parts.
  • the method comprises forming a metal nitride or metal alloy nitride on a metal or metal alloy surface of a gas turbine component by covering the surface with an oxygen-reacting composition comprising (a) particles of a metal that oxidizes in preference to the metal or metal alloy of the surface and that is unreactive with respect to the metal or metal alloy of the surface and (b) particles of an inert material and exposing the covered surface to industrial grade nitrogen at an elevated temperature and for a time sufficient to produce a metal nitride or metal alloy nitride on the surface,
  • the titanium and titanium alloys used in this study are commercially pure Ti (Grade 2) and Ti-6Al-4V alloy.
  • the composition of the commercially pure Ti and Ti6A14V is shown in Table 1. Small blocks with the dimensions of 10 x 10 x10 mm 3 were cut from commercial plates of pure Ti and Ti-6A1-4V alloys, followed by mechanical grinding up to grade 4000 on silicon carbide paper. Table 1 Compositions of commercial pure Ti (Grade 2) and Ti-6A1-4V alloy in wt.%.
  • oxygen-reacting compositions consist of metal particles with a balance of fused A1 2 0 3 (AWA, ⁇ 45 ⁇ ).
  • the metal particles used were commercial magnesium powder (Atomized Mg Powder, ⁇ 45 ⁇ ) or commercial copper powder (Cu 101 , Copper Metal Flake, 99.9%, 1 -5 ⁇ ).
  • the composition is prepared by thorough mixing for 30 minutes in a mechanical power-mixer. Gas nitriding procedure used in the examples
  • Heating takes place in a small stainless steel container placed in a GSL 1600X tube furnace, which can work continuously at a maximum temperature of 1500°C.
  • the furnace is sealed, evacuated to a pressure of -0.1 MPa and backfilled with nitrogen.
  • the evacuation-backfill process is repeated three times to minimize atmospheric air in the furnace chamber. Pure nitrogen with industrial purity (min 99.5%) is used instead.
  • the furnace is then heated using a heating rate of 5 °C/min to different target temperatures, usually between 650°C and 1050°C. Once the target temperature is reached, the furnace chamber is kept under constant pressure (0.04 MPa) using a nitrogen flow rate (industrial grade) of 0.1-0.3 ml/min, and the temperature maintained for up to 24 hours. After gas nitriding, the samples were cooled to room temperature under the same nitrogen atmosphere before removal from the furnace. Microstructure examinations
  • the samples were cut into two small blocks with dimensions of 10 x 10 x 5 mm. Cross-sections of these blocks, perpendicular to the surface, were mechanically ground using silicon carbide paper up to grade 4000. Then, a final polish was performed on a polishing cloth using a liquid suspension of 0.05 ⁇ alumina, followed by ultrasonic cleaning and drying.
  • TEM Transmission Electron Microscope
  • X-ray diffraction was used to characterize the nitride layers formed on pure Ti and Ti-6AI-4V substrates.
  • the XRD analysis was carried out using a Bruker D8 diffractometer operated at 40 kV with A).
  • the micro-hardness of the surface and cross-sectional depth variation from the nitrided surface to the substrate core was determined using a Struers DURAMIN 20 hardness testing machine.
  • the micro-hardness test was carried out using a load of 100 g on the surface, and 50 g on the sample cross-section. The load was applied for 12 s.
  • Dry sliding wear tests on pure Ti and Ti-6A1-4V surface after gas nitriding were performed on an Optimol SRVIII oscillating friction and wear tester at room temperature (25°C) in air with a relative humidity of 40-50%.
  • the machine was equipped with a ball-on-disc contact configuration, which uses tungsten carbide - cobalt balls with a hardness of HV- 1750 as the counter friction pair.
  • Prior to testing all specimens were cut into 10 10 x 3 mm plates, and then the cutting plane was mechanically polished up to grit 1200 parallel to the nitride surface, followed by ultrasonic cleaning and air drying.
  • Corrosion resistance of pure Ti and Ti-6A1-4V after gas nitriding was assessed visually using salt spray testing, and electrochemically using linear potentio-dynamie polarization, both in a neutral 5 wt.% NaCl solution.
  • as-received samples of pure Ti, Ti-6A1-4V and 316 stainless steel were also used in both tests.
  • the salt spray tests were conducted as per the ASTM B l 17 standard. During the tests, all samples were placed in a chamber at a temperature of 35°C under an aqueous spray (pH between 6.5 and 7.2). These samples were photographed at regular intervals by a digital camera, and then were cleaned and grit blasted with low air pressure to clear the surface corrosion product. The weight gain/loss was measured by an electronic balance to 0.0001 g as a function of spray duration.
  • FIG. 1 shows cross-section optical micrographs of a pure Ti sample after nitriding at 950°C for 24 h in nitrogen flow, in the absence (a) and in the presence of (b) a powder mixture of A1 2 0 + 5 wt.% Mg.
  • FIG. 3(a)-(c) show the nitride layer at temperature from 650°C to 750°C. At higher temperature the thickness of the nitride layer significantly increased. At 850°C, a 12- ⁇ thick layer was observed after nitriding for 24 h as shown in the optical microscope image of Figure 9(e).
  • Figure 3(f) shows a plot of the nitride layer thickness observed after 24 hours at a temperature in the 650-950°C range. Nitriding temperature can promote the formation of nitride layer, especially above 800°C. At 950°C, the thickness of the nitride layer was 40 ⁇ after nitriding for 24 h.
  • FIG. 3 shows the X-ray diffraction patterns of pure Ti after nitriding at 650°C (a) and 800°C (b) for 24 h in a powder mixture of A1 2 0 3 + 5 wt.% Mg.
  • the XRD patterns indicate that the titanium nitride layers comprise TiN and Ti 2 N.
  • TEM investigations were performed to spatially identify the nitride layers.
  • Figure 5 shows that the nitride layers contain two interfaces, which are recognisable in bright field mode. Diffraction patterns of the corresponding layers clearly clarified that the outer sub-layer is TiN and the inner one is Ti 2 N.
  • Nitriding of pure titanium at 950°C for different annealing times has been carried out in a powder mixture of Al 2 (1 ⁇ 2 + 5 wt.% Mg.
  • Figure 6 shows cross-section optical micrographs of pure Ti samples after gas nitriding in a powder mixture of A1 2 0 3 + 5 wt.% Mg at 950°C for 12 h (a) and 24 h (b).
  • the thickness of the nitride layers is about 24 ⁇ after nitriding for 12 h, whilst it almost doubled after nitriding for 24 h at the same temperature, as shown in Figure 6(b). This indicates that the nitride layer thickness can also be increased by prolonging the nitriding time, besides raising the nitriding temperature.
  • Cu metal particles were used instead of Mg particles.
  • a powder mixture of A1 2 0 3 + 60 wt.% Cu was used to purify the industrial grade nitrogen during the nitriding of pure Ti samples at temperatures from 850°C to 1050°C, for up to 24 h.
  • Figure 7 shows the optical micrographs of the cross-section of the pure titanium samples after gas nitriding in a powder mixture of A1 2 0 3 + 60 wt.% Cu at different temperature from 850°C to 1050°C for 24 h. It can be seen that Cu powder can also protect the substrate from oxygen, thus favouring the formation of a nitride layer on its surface, similarly to what is obtained using Mg particles.
  • the thickness of nitride layer after nitriding for 24 h increases slowly below 900 °C, but significantly above 950 °C.
  • a 150- ⁇ thick nitride layer can be achieved at 1050 °C after 24 h ( Figure 7(e)).
  • Figure 8 shows optical micrographs of the cross-section of pure Ti samples after gas nitriding in a powder mixture of Al 2 0 3 + 60 wt.% Cu at 950 °C for 12 h and 24 h.
  • a 23- ⁇ thick nitride layer can be achieved on pure Ti at 950 °C for 12 h using a powder mixture of Al 2 O 3 +60 wt.% Cu, which is similar to the thickness obtained under the same conditions using an Al 2 0 3 +5 wt.% Mg powder mixture.
  • gas nitriding of titanium alloys should be performed at a temperature below their ⁇ -transus temperature.
  • the A1 2 0 3 + 5 wt.% Mg powder mixture is more suitable for pure Ti other than the powder mixture of A1 2 0 3 + 60 wt.% Mg.
  • Example 6 A micro-hardness test was performed on pure Ti samples after nitriding in a powder mixture of A1 2 0 3 + 5 wt.% at temperatures from 650°C to 850°C for 24 h.
  • Figure 9 shows the micro-hardness profiles on the surface and the cross-section of pure Ti samples after gas nitriding in a powder mixture of A1 2 0 3 + 5 wt.% Mg at temperatures from 650°C to 850°C for 24 hours.
  • a micro-hardness of HVo.i 1650 can be achieved on titanium surface after nitriding at 850°C for 24 h, as result of the thick nitride layer shown in Figure 3(e).
  • the nitrided titanium and nitrided titanium alloy samples have a similar wear resistance, indicated by worn depth and volume loss in Figure 1 1 , when using a load below 40 N.
  • the load exceeds 40 N, the nitrided pure Ti sample exhibits higher wear resistance than the nitrided Ti-6A1-4V sample.
  • SEM observations show that the nitride layer on the Ti-6A1-4V sample was worn out at 40 N, which leads to a similar morphology of worn scar with as-received Ti-6A1-4V.
  • the nitride layer on the pure Ti sample shows a typical morphology on worn scar compared to as-received Ti-6A1-4V under a 50 N load. This is attributed to a thicker nitride layer produced on pure Ti than that on Ti-6A1-4V.
  • nitride layers on both pure Ti and Ti-6A1-4V exhibits an excellent wear resistance, they have the similar average coefficient of friction compared to as-received Ti-6Al-4V. This is probably due to the high surface roughness caused by gas nitriding.
  • Figure 13 shows macrographs of nitrided pure Ti and Ti-6A1-4V samples surface exposed to neutral salt spray for 180 h and 326 h, compared with 316 SS, pure Ti and Ti-6A1-4V substrates. No visible corrosion attack is observed. The weight gain/loss recorded at regular intervals is shown in Figure 14. No obvious weight change is detected in all samples exposed to salt spray for 326 h. The fluctuation of curve is caused by the salt remaining on samples.
  • the response of the nitride layer subject to linear polarization is shown in Figure 15, plotted with respect to the standard Ag/AgCl- reference electrode potential. It can be seen that the nitride layer has a significant effect on the polarization behaviour of the titanium and titanium alloy substrates. Although the corrosion current density remains the same for both pure Ti and Ti-6A1-4V before and after nitriding, the corrosion potential significantly increases after nitriding. This indicates that the nitriding can improve the corrosion resistance of titanium alloys as well.

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  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

Abstract

L'invention porte sur un procédé de formation d'un nitrure métallique ou d'un nitrure d'alliage métallique sur un substrat en métal ou en alliage métallique correspondant, lequel procédé comprend : le recouvrement du substrat d'une composition réagissant avec l'oxygène comprenant (a) des particules d'un métal qui s'oxyde en priorité par rapport au substrat en métal ou en alliage métallique et qui n'est pas réactif avec le substrat en métal ou en alliage métallique et (b) des particules d'un matériau inerte ; et l'exposition du substrat recouvert à de l'azote de qualité industrielle à une température élevée et pendant une durée suffisante pour produire le nitrure métallique ou le nitrure d'alliage métallique sur le substrat.
PCT/AU2012/001544 2011-12-22 2012-12-17 Procédé de traitement WO2013090987A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5538966A (en) * 1978-09-14 1980-03-18 Hitachi Ltd Nitriding method for titanium and titanium alloy
JPH08269682A (ja) * 1995-03-31 1996-10-15 Toyota Motor Corp アルミニウムの表面窒化処理方法、窒化処理用助剤および表面窒化アルミニウム材
US6074494A (en) * 1995-10-02 2000-06-13 Toyota Jidosha Kabushiki Kaisha Surface nitriding method of an aluminum material, and an auxiliary agent for nitriding

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5538966A (en) * 1978-09-14 1980-03-18 Hitachi Ltd Nitriding method for titanium and titanium alloy
JPH08269682A (ja) * 1995-03-31 1996-10-15 Toyota Motor Corp アルミニウムの表面窒化処理方法、窒化処理用助剤および表面窒化アルミニウム材
US6074494A (en) * 1995-10-02 2000-06-13 Toyota Jidosha Kabushiki Kaisha Surface nitriding method of an aluminum material, and an auxiliary agent for nitriding

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