METHOD OF MODIFYING THE SURFACE OF A SUBSTRATE

Abstract: To improve the surface properties of a substrate selected from the group consisting of the metals titanium, zirconium and hafnium and alloys based on any of these metals, a solid material such as silicon carbide is dissolved in a molten layer at the surface of the substrate, and the molten layer is allowed to cool rapidly. The silicon dissolves in the liquid titanium to produce a substitutional solid solution which may then at least partially precipitate out upon cooling, whilst the carbon initially dissolves to produce an interstitial solid solution but on cooling may at least partially precipitate out to form a fine distribution of titanium carbide.

Description: METHOD OF MODIFYING THE SURFACE OF A SUBSTRATE This invention relates to a method of modifying the surface of a metal substrate to impart improved properties thereto. The present invention is particularly applicable to substrates of titanium, zirconium or hafnium or an alloy based on any of these metals, ie an alloy in which such metal is present in an amount greater than any one of the other components of the alloy. Although titanium is strong and light, applications of titanium in general engineering are limited by its poor tribological properties. It is therefore desirable to provide a hardened layer from 0.1 to 2 mm deep on the surface of titanium or a titanium alloy in order to enhance its wear properties. It has been proposed to improve the hardness and wear resistance of titanium by making alloy additions to the surface layer thereof. It has also been proposed to produce significant alloy additions to depths of around 1 millimetre by scanning the surface with a high energy beam such as a laser or electron beam in an atmosphere containing nitrogen and/or carbon, so as to produce a molten pool of titanium in which nitrogen and/or carbon become dissolved. Quenching of the molten pool containing the dissolved nitrogen and/or carbon takes place as a result of the presence of the main mass of the relatively cool substrate. It has also been proposed to incorporate carbon by carbide particle injection. It is an object of the present invention to provide an improved method of modifying the surface of a substrate of titanium, zirconium or hafnium or an alloy based on any of these, which can lead to improved surface properties, in particular improved wear resistance and load bearing capacity. According to the present invention, there is provided a method of modifying the surface of a substrate selected from the group consisting of the metals titanium, zirconium and hafnium and alloys based on any of these metals, said method comprising the steps of dissolving a solid material in a molten metallic layer at the surface of the substrate, said solid material containing at least one interstitial alloying element which is capable of entering into interstitial solution in a phase in the metallic layer, and at least one substitutional alloying element which is capable of substituting, in the metallic layer, a portion of said metal of which the substrate is based; and thereafter cooling the metallic layer whereby, upon solidification, there is formed at the surface of the substrate a solid metallic layer containing said at least one interstitial alloying element interstitially and said at least one substitutional alloying element substitutionally. Upon further cooling, said solid metallic layer may undergo solid state reactions and as a consequence after cooling, the or each interstitial alloying element may be in solid solution and/or in a precipitate, and the or each substitutional alloying element may also be in a solid solution and/or in a precipitate Also according to the present invention, there is provided a component comprising a substrate selected from the group consisting of the metals titanium, zirconium and hafnium and alloys based on any of these metals, said substrate having a surface layer formed by melting and cooling at least a part of the surface of the substrate, said surface layer containing (a) at least one interstitial alloying element which has entered into interstitial solution in. a phase in said layer and. which has optionally at least partially precipitated from solution during cooling of the surface layer, and (b) at least one substitutional alloying element which has substituted a portion of the metal of which the substrate is based and which has optionally at least partially precipitated from solid solution during cooling of the surface layer. In order to increase the concentration of alloying elements in the molten pool, a second application of the solid material may be made after the molten surface has solidified and a second melting process carried out. Alternatively, a second melting process may be carried out without recourse to a second application of solid material and in this way it is possible to improve the smoothness of the as-solidified surface. It will be appreciated that the method of the present invention involves the use of a solid material which provides alloying elements serving two functions, and therefore capable of effecting complementary mechanisms to strengthen and enhance the wear resistance of the surface. Said at least one interstitial alloying element is preferably selected from the group consisting of boron, carbon, nitrogen and oxygen. Said at least one substitutional alloying element is preferably selected from the group consisting of silicon, chromium, manganese, iron, cobalt, nickel and copper, each of these elements being a beta-stabilizing alloying addition capable of entering into substitutional solid solution in the substrate and also capable of forming an intermetallic compound with the principal element of the substrate. The solid material containing said at least one interstitial alloying element and said at least one substitutional alloying element is conveniently silicon carbide or a mixture of silicon carbide and silicon, but may be silicon nitride or chromium nitride, for example. The amount of said at least one substitutional alloying element included in the molten layer is preferably from 5 atomic?, up to 25 atomic?., more preferably 10 atomic% to 15 atomic%. Where the substitutional alloying element is silicon and the substrate is titanium or a titanium based alloy, this corresponds approximately to a range of preferably 3 to 16 weight?., more preferably 6 to 10 weight?., of silicon in the molten layer. The atomic ratio of said at least one substitutional alloying element to said at least one interstitial alloying element in the molten layer is preferably 1:3 to 3:1, more preferably 1:1 to 3:1. In the case where silicon carbide is used as the solid material and the metallic layer is titanium or an alloy based on titanium, it is believed that the silicon from the silicon carbide dissolves in the liquid titanium which solidifies as a substitutional solid solution of body-centred cubic beta phase. Small amounts of primary titanium suicide may also be formed on solidification. On subsequent cooling and phase transformation, some of the silicon remains in solid solution and some of it is precipitated as fine particles of titanium suicide, Ti 5 - Si 3 . It is believed that the carbon from the silicon carbide initially dissolves to produce an interstitial solid solution but on cooling precipitates out to form in addition a fine distribution of titanium carbide which is present principally as a constituent of a eutectic phase but which may also appear as carbide dendrites. The combination of solid solution of the two addition elements together with the formation of precipitated compounds containing one or both of the addition elements produces a hardened surface layer having improved resistance to wear, particularly under conditions of high contact load and either rolling or mixed rolling and sliding contact. The method is preferably conducted so that the minimum depth of the modified layer is 0.1 mm. The maximum depth is usually about 2 mm. , with the preferred depth being of the order of 1 mm. The solid material may be provided on the substrate by any desired means such as by painting or spraying of a solid material slurry or suspension which may or may not contain a temporary binder. The applied layer is allowed to dry before surface melting with a high energy beam such as a scanning laser or electron beam. The power and rate of scan of the high energy beam can be chosen to effect the desired degree of melting at the surface. Instead of using a laser or electron beam, it is within the scope of the present invention to effect heating using an electric arc such as that provided by a tungsten-inert gas torch. It is important only to melt the surface layer portion of the substrate. Maintaining a relatively cool substrate not only ensures fast quenching of the molten layer, but also ensures that any desirable microstructure which has been previously developed in the substrate will not be lost through o* --.rheating of the latter. The present invention will now be described in further detail in the following Examples:- Example 1 A layer of silicon carbide was applied to the surface of a titanium alloy (Ti-6A1-4V) having a thickness of 12 mm. The silicon carbide was applied by painting a slurry of commercially available silicon carbide powder having an average particle size of 10 μm and drying it to provide a dense silicon carbide layer in intimate contact with the surface of the substrate. The coating weight of silicon carbide ranged from 200-1000 g (e.g. typically 400-500 g) per square metre of surface area. The thus coated substrate was then subjected to surface melting using a scanning electron beam provided by a machine sold by Carl Zeiss under the identification ES 1013. The electron beam employed had a focused spot of 0.2 mm. diameter with beam parameters of 100 kW and 3 mA which was traversed at a feed rate of 10 to 200 mm/s to provide tracks of 1 to 0.1 mm. deep with a good surface finish. During traverse of the electron beam, localised melting of the surface layer of the titanium substrate occurred under the silicon carbide layer to form a small pool in which dissolution of the silicon carbide took place with convective mixing. After passage of the traversing beam, solidification of the molten pool occurred rapidly as a result of heat transfer to the relatively cool main body of the substrate under the pool. The depth of melting could be chosen to suit the intended requirements by appropriate selection of the power and/or traverse rate of the high energy beam. The degree of alloying could be chosen by appropriate selection of the ratio of coating thickness to total melt depth. In one series of experiments, the microhardness of the surface alloyed layer was in the range 650 to 700 H v and the microstructure consisted of finely divided primary titanium carbide, a major constituent of alpha titanium containing silicon in solution and a eutectic phase. Substantial amounts of the intermetallic compound Ti 5 -Si 3 were identified by X-ray diffraction. This microstructure was compared with the microstrucure of bulk alloys prepared by melting titanium, silicon and carbon, which demonstrated that the composition of the alloyed surface contained in the region of 8.5 weight% silicon and 3.6 weight?, carbon. In another series of experiments, the lubricated wear resistance of a disc of commercial titanium alloy (Ti-6A1-4V) which had been surface alloyed with silicon and carbon (1:1 atomic ratio) according to the invention was compared with untreated alloy and with EN 19 steel. Before testing, the alloyed surface had been machined and ground to give a 0.5 μm R a finish, the depth of alloyed layer measured as 200 to 300 μm and the microhardness measured in the range of 650 to 700 H v . When tested under conditions of rolling with 30% sliding, under an applied load of 13 kg, a steady state wear rate was achieved with the surface alloyed disc for an extended period which was one hundredth of that of the untreated alloy and was approximately equal to that a hardened steel surface. When the same test was carried out with an applied load of 120 kg, a steady state wear rate was again achieved, although for a shorter period, and this was one thousandth of that of the untreated alloy and again approximately equal to that of hardened steel. Example 2 Example 1 was repeated using chromium nitride in place of silicon carbide. An alloyed layer was produced having a depth of 0.3 to 0.4 mm and a microhardness in the range of 700 to 800 H„.

Claims: CLAIMS 1. A method of modifying the surface of a substrate selected from the group consisting of the metals titanium, zirconium and hafnium and alloys based on any of these metals, said method comprising the steps of dissolving a solid material in a molten metallic layer at the surface of the substrate, said solid material containing at least one interstitial alloying element which is capable of entering into interstitial solution in a phase in the metallic layer, and at least one substitutional alloying element which is capable of substituting, in the metallic layer, a portion of said metal of which the substrate is based; and thereafter cooling the metallic layer whereby, upon solidification, there is formed at the surface of the substrate a solid metallic layer containing said at least one interstitial alloying element interstitially and said at least one substitutional alloying element substitutionally. 2. A method as claimed in claim 1, wherein, upon further cooling, the or each interstitial alloying element is in solid solution and/or in a precipitate, and the or each substitutional alloying element is also in a solid solution and/or in a precipitate. 3. A method as claimed in claim 1 or 2, wherein, in order to increase the concentration of the alloying elements in the molten pool, a second application of the solid material is made after the molten surface has solidified and a second melting process carried out. 4. A method as claimed in claim l"or 2, wherein a second melting process is carried out without any further application of the solid material. 5. A method as claimed in any preceding claim, wherein said at least one interstitial alloying element is selected from the group consisting of boron, carbon, nitrogen and oxygen. 6. A method as claimed in any preceding claim, wherein said at least one substitutional alloying element is selected from the group consisting of silicon, chromium, manganese, iron, cobalt, nickel and copper. 7. A method as claimed in any preceding claim, wherein the solid material is added in an amount such that said at least one substitutional alloying element is present in the molten layer in an amount of from 5 up to 25 atomic?.. 8. A method as claimed in claim 7, wherein the solid material is added in an amount such that said at least one substitutional alloying element is present in the molten layer in an amount of from 10 to 15 atomic%. 9. A method as claimed in any preceding claim, wherein the atomic ratio of said at least one substitutional alloying element to said at least one interstitial alloying element is 1:3 to 3:1. 10. A method as claimed in any preceding claim, wherein the thickness of the surface layer is 0.1 to 2 mm. 11. A component comprising a substrate selected from the group consisting of the metals titanium, zirconium and hafnium and alloys based on any of these metals, said substrate having a surface layer on at least a part thereof, said surface layer containing (a) at least one interstitial alloying element which has entered into interstitial solution in a phase in said layer and which has optionally at least partially precipitated from solution, and (b) at least one substitutional alloying element which has substituted a portion of the metal of which the substrate is based and which has optionally at least partially precipitated from solid solution.

Inventors: BELL THOMAS, BLOYCE ANDREW, MORTON PETER HARLOW, BELL, THOMAS, BLOYCE, ANDREW, MORTON, PETER, HARLOW
Applicants: UNIV BIRMINGHAM, THE UNIVERSITY OF BIRMINGHAM
Application year: 1990
Main classification: C23C12
IPC codes: B23K35, C23C12, C23C26