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

所谓的MAX相包括形成化合物的三种元素。M是前过渡金属（Sc、Ti、V、Cr、Zr、Nb、Mo、Hf、Ta）中的一种，A通常是III A或IV A族元素（Al、Si、P、S、Ga、Ge、As、Cd、In、Sn、Tl、Pb）中的一种，但也可包括Cd、P、S或As，X是C或N，其添加到先前选择的两种元素中。它们的一般形式是Mn+1AXn，其中n为1、2或3，并且取决于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年代由Nowotny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，当它被发现，由Barsoum和他的同事，这些高延展性材料（Ti 3SiC 2）是抗热震和抗氧化，即使在超过1000 ℃的温度。这种涂层的潜在应用范围很广，它们可以应用于切削工具、端米尔斯、高温下工作的机械部件、涡轮机叶片和输送高温流体的管道。它们的成分和层状结构使它们可用于耐腐蚀和耐辐射的应用，并可能用作核工业中的包覆层。传统上，这些材料是在约1400 ℃的温度下从本体合成的，但薄膜Ti 3SiC 2是在1200 ℃下通过化学气相沉积（CVD）形成的，这种化合物和其他相可以在900 ℃下形成。1000 oC的温度下，使用磁控溅射在合适的基板上。然而，对于经济上可行的工业过程，需要降低过程温度。许多基材在高温下不稳定，例如含碳量低至0.25%的钢在723 ℃时发生相变。本提案的主要目的是开发在低于700 ℃的温度下制备基于Ti-Si-C和Ti-Al-N的薄膜MAX相的技术。层组分的顺序沉积和在生长的膜表面以高能粒子碰撞的形式添加能量。顺序沉积将通过使用溅射方法切换源靶或通过在CVD工艺中脉冲不同的前体气体来实现。在沉积条件方面也存在需要解决的问题，杂质可能会阻止MAX相的正确成核，可能需要单晶衬底，并且可能需要籽晶层。在形成薄MAX相层后，将使用高分辨率显微镜分析其正确的结构特性，并通过测量硬度和摩擦来分析其机械特性。热稳定性将从热循环实验中判断。这些涂料具有很高的工业兴趣，Teer Coatings Ltd和Applied Multilayers Ltd正在支持这一计划。

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.