Metal alloys, such as titanium alloys and steels, are known to have a good combination of mechanical properties for many structural applications, but these metal alloys do not meet the wear and corrosion resistance requirements for some structural applications. Titanium alloys, for example, have many attractive properties such as high specific strength and stiffness, relatively low density, and excellent corrosion resistance, but have poor resistance to wear and oxidation at high temperatures. Conventional surfacing (such as nitriding), coating deposition (such as plasma spraying and sputtering), and plating have significant shortcomings that include potentially providing distorted substrates, deteriorated surfaces, and/or weak interfacial bonding.
To overcome these shortcomings and provide high wear- and corrosion-resistant surfaces on metal alloy substrates, surface alloying and reactive surface modification have been developed — depositing and post-heat-treating a unique combination of materials, selected based upon the substrate material and specific application environment. Functionally graded or layered interfaces are used to overcome interfacial bonding weaknesses, especially when the coefficient of thermal expansion (CTE) is significantly different between the substrate and a ceramic or cermet surface coating.
Hardface coating systems for metal alloys and other materials provide wear and corrosion resistance to overcome some of these attendant shortcomings. A titanium boron coating is applied as a liquid to the surface of another metallic object. As the liquid cools, it bonds to the surface and undergoes a chemical reaction that provides superior wear and thermal stress properties. The metallic coating combines the performance of heat treatment and alloys, bonded to the surface of the substrate.
The coatings are applied by deposition processes including thermal spraying, physical vapor deposition (PVD), powder coating followed by post-heat treatment, as well as by slurry coating. The reinforced composite structure in the coatings is preferably formed during the thermal spray or PVD processes, whereby the process heat provides the inherent energy to facilitate the reaction for the formation of the desired composition. In such cases, post-heat treatment is not necessary; however, in some cases, such post-heat treatment may increase the percentage of reinforcement in the matrix.
Surface engineering, surface modification, or surface alloying of the substrate metal alloys may be accomplished by depositing a slurry, suspension, blend, or mixture of selective materials onto a surface using a number of methods such as painting, spraying, thermal spraying, dipping, powder coating, etc., and then reactively forming the surface by heating using laser radiation, plasma radiation, infrared radiation, electron beam radiation, microwave radiation, induction, welding, etc.
The surface coatings formed by this approach change the surface characteristics of a component or structure to provide properties of high hardness, high temperature strength, wear and corrosion resistance, and strong adherence to the substrate without significantly changing the bulk material properties. Layers or functional grading may be employed to increase bonding strength and adherence, or to mitigate differences in the CTE, whether or not post-heat treatment is utilized. The surface may be applied to finished components by portable field techniques, or fabricated onto sheet materials prior to the final manufacturing steps.