A study of methods to produce MAX phases in thin films at temperatures below 700 degrees C

所谓的MAX相是由三种元素组成的化合物。M是早期过渡金属之一（Sc, Ti， V, Cr, Zr, Nb, Mo, Hf, Ta）， A通常是III A或IV A族元素之一（Al, Si， P， S, Ga, Ge, As, Cd, In, Sn, Tl, Pb），但也可以包括Cd， P， S或As， X是C或N，这是先前选择的两个元素的补充。它们的一般形式是Mn+1AXn，其中n为1、2或3，取决于a元素层之间MX块的不同堆叠顺序。它们最初是在20世纪60年代被诺沃特尼等人发现的，他们研究了它们的结构。工程性能一直被忽视，直到最近，Barsoum和他的同事发现，其中一种高延展性材料（Ti3SiC2）即使在超过1000摄氏度的温度下也能抗热冲击和抗氧化。因此，这些材料既具有陶瓷特性，又易于加工。这种涂料的潜在应用是广泛的；它们可以应用于切削工具，立铣刀，在高温下运行的机器部件，涡轮叶片和携带高温流体的管道。它们的组成和分层结构使它们在抗腐蚀和抗辐射应用中很有用，可能在核工业中用作包层。传统上，材料是在1400℃左右的温度下从体体合成的，但薄膜Ti3SiC2已经在1200℃的温度下通过化学气相沉积（CVD）形成，这种化合物和其他相可以在900-1000℃的温度下在合适的衬底上使用磁控溅射形成。然而，对于经济上可行的工业过程，过程温度需要降低。许多衬底在高温下是不稳定的，例如，钢的温度低至0.25%，在723℃时发生相变。该提案的主要目的是开发在低于700℃的温度下生产基于Ti-Si-C和Ti-Al-N的薄膜MAX相的技术。有两种新方法有待探索：层组分的连续沉积和在生长的薄膜表面以高能粒子碰撞的形式增加能量。顺序沉积将通过使用溅射方法切换源靶来实现，或者通过在CVD过程中脉冲不同的前驱体气体来实现。在沉积条件方面也有需要解决的问题，杂质可能会阻止MAX相的正确成核，可能需要单晶衬底和播种层。在形成薄的MAX相层后，将使用高分辨率显微镜分析其正确的结构性能，并通过测量硬度和摩擦来分析其机械性能。热稳定性将通过热循环实验来判断。该涂料具有很高的工业价值，蒂尔涂料有限公司和应用多层有限公司正在支持该计划。

The so-called MAX phases comprise three elements formed into a compound. M is one of the early transition metals (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta), A is usually one of the group III A or IV A elements (Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Tl, Pb) but can also include Cd, P, S or As and X is either C or N which adds to the previously selected two elements. Their general form is Mn+1AXn where n is 1,2 or 3 and depends on the different stacking sequence of the MX block between the A-element layers. They were initially discovered in the 1960's by Nowotny et al who studied their structure. Engineering properties have been neglected until recently when it was found, by Barsoum and his colleagues, that one of these highly ductile materials (Ti3SiC2) was thermal shock resistant and oxidation resistant even at temperatures in excess of 1000oC . Thus these materials have ceramics properties yet are easy to machine.The potential application of such coatings is wide-ranging; they can be applied to cutting tools, end mills, machine components operating at high temperature, turbine blades and pipelines carrying high temperature fluids. Their composition and layered structure makes them useful for corrosion- and radiation-resistant applications with possible application as cladding layers in the nuclear industry.Conventionally the materials have been synthesised from the bulk at temperatures about 1400oC but thin film Ti3SiC2 has been formed at 1200oC by Chemical Vapour Deposition (CVD) and this compound and other phases can be formed at 900-1000oC on a suitable substrate using magnetron sputtering. For an economically viable industrial process however the process temperature needs to be reduced. Many substrates are not stable at high temperatures, for example steel, containing as low as 0.25% C, undergoes a phase transition at 723oC.The main thrust of this proposal is to develop techniques which will produce thin film MAX phases based on Ti-Si-C and Ti-Al-N at temperatures below 700oC.There are 2 novel approaches to be explored: sequential deposition of the layer components and addition of energy in the form of energetic particle collisions at the growing film surface. The sequential deposition will be achieved by switching source targets using sputtering methods, or, by pulsing different precursor gases in the CVD process. There are also concerns to be addressed in terms of deposition conditions, impurities can prevent correct nucleation of the MAX phases, single crystal substrates may be needed and a seeding layer could be required. Having formed the thin MAX phase layers they will be analysed for their correct structural properties using high resolution microscopy and for mechanical properties by measuring hardness and friction. Thermal stability will be judged from thermal cycling experiments. The coatings have high industrial interest and Teer Coatings Ltd and Applied Multilayers Ltd are supporting this programme.