N掺杂对磁控溅射Ta涂层微观结构与耐磨损性能的影响

Effect of Nitrogen Doping on Microstructure and Wear Resistance of Tantalum Coatings Deposited by Direct Current Magnetron Sputtering

采用磁控溅射技术,以AISI304不锈钢为基体,通过在溅射过程中引入不同流量的N2,制备出不同程度N掺杂的Ta涂层,研究了少量N掺杂对Ta涂层微观结构、物相组成、力学及磨损性能的影响。结果表明,无N掺杂时,Ta涂层中的物相组成为α-Ta,晶粒粗大,(211)和(110)晶面竞争生长;N掺杂后涂层中的物相组成为TaN0.1,晶粒细小并呈(110)择优取向。少量N掺杂可以显著提高Ta涂层的硬度和弹性模量,并大幅度改善其抗磨损性能。涂层硬度和弹性模量的提高与晶粒细化、N原子固溶及涂层中存在的压应力有关。N掺杂后涂层的磨损机制由磨粒磨损向黏着磨损转变。

Tantalum coating attracts increasing attention in heat, corrosion and wear resistant applications today because of its high melting point, immunity to chemical attack and high toughness. Recently, tantalum has been considered a desirable candidate to replace electrodeposited(ED) chromium coating which is often used as protective coating against corrosion and wear. However, the wastes associated with ED chromium contain a well-known carcinogen, i.e. hexavalent chromium, which is a hazard to environment. In comparison, thick Ta coating is regarded as a more environmental and beneficial replacement. Tantalum coating is usually obtained by magnetron sputtering. However, tantalum exhibits two distinct crystalline phases. The body-centered cubic α-phase is the common phase in bulk metal and thermodynamically stable. α-Ta with good ductility and excellent mechanical properties is welcomed in most fields. β-Ta is a metastable phase with tetragonal crystalline lattice structure. The properties of β-Ta are not as advantageous as α-Ta because it is hard and brittle. The existence of β-Ta may compromise tantalum coating in adhesion, corrosion and wear resistance, hence, finding appropriate deposition conditions to obtain pure α-phase Ta coating has attracted a lot of interests. In previous work, pure α-phase Ta coating has been deposited by direct current magnetron sputtering when substrates were located in negative glow space. In this work, nitrogen was mixed in sputtering gases to deposit Ta coating with N interstitially dissolved on stainless steel. Effect of N on microstructure, mechanical and tribological performance of Ta coating was studied. Results indicated that when no nitrogen or very low flux of N2(l mL/s) were introduced in gas mixtures, α-phase Ta coating with coarse grains grew and revealed strong reflections of(211) and(110) diffraction peaks. When N2 flow rate reached to 5 mL/s, Ta coating with N interstitially dissolved was obtained and revealed grain refinement and(110) preferred orientation of TaN0.1 phase. Compared to α-phase Ta coating, N-doped tantalum coatings displayed excellent wear resistance for their high hardness and H3/E2 ratio(H—hardness, E—elastic modulus). The wear mechanism for α-Ta coating was abrasive wear, while that of N-doped Ta coating switched to adhesive wear.