MoSi2涂层制备对于双组元离心发动机的影响分析

Effect of the MoSi Coating Preparation Quality on Performance and Reliability of Bipropellant Centrifugal Engine

涂覆在双组元发动机推力室表面的MoSi2涂层可以有效地防止金属基材在高温氧化氛围下形成低熔点的金属氧化物。通过真空离子镀和包渗硅化的制备方法,可以令MoSi2涂层更加致密和均匀,大大提高了涂层性能。其涂层试片在1800℃的静态高温下可以耐受20 h以上,远远优于传统挂浆和喷涂法制备的试片耐温指标。同时,该工艺方法制备的涂层表面更加光洁致密,在这种条件下更容易令冷却液膜铺展和生存,也更有利于实现液体蒸发换热,以实现发动机燃烧室头部的高效冷却。经实验验证,涂覆了该涂层的发动机在喉部1400℃以上的点火温度下,头部温度仅约100℃,并顺利通过了累计4万秒的寿命摸底考核,这对于发动机性能提升和延寿有重要意义。但是,在MoSi2涂层的制备过程中,钼层的厚度和均匀性控制以及包渗硅化工艺的匹配性都会对发动机可靠性有着重要影响。一旦有钼层残留在涂层内部,就会导致涂层扩散层结构异常,严重影响涂层的结合性和热匹配性,在点火中产生贯穿性裂纹而失效。失效位置往往位于推力室喉部下游,因为该位置较大的温度梯度和相对贫氧的环境导致涂层内部产生应力裂纹并难以自愈合。针对这个问题,必须通过控制钼层厚度、延长包渗时间和称重法检测等措施,以保证钼层足以形成足够厚度的涂层,同时实现完全的硅化而没有残留。通过这些措施可以提高涂层制备质量,保证发动机的工作可靠性。

The MoSi 2 coating on the chamber surface of the bipropellant engine is mainly used to prevent metal substrate from forming low-melting metal oxide under high-temperature oxidation atmospheres in the firing process.The MoSi 2coating is made by vacuum ion plating and infiltration siliconization.In this way,it can make the coating much denser and more uniform,getting the better performance to keep effective under the harsh conditions of high temperature and scouring.In the static high-temperature test,the sample with MoSi 2 coating can withstand more than 20 hours under the of 1800 degrees Celsius,which is far better than the temperature resistance of the other sample made by the traditional slurry or spray method.Meanwhile,the MoSi 2coating surface based on vacuum ion plating and infiltration siliconization is smoother and denser.Experiments show that under these conditions,the cooling liquid film is easier to spread and survive and has better evaporative heat transfer effect,which is conducive to achieving highly efficient cooling for weakening the heat flux to the engine injector.It is verified in the firing test that the bipropellant centrifugal engine covered with MoSi 2 coating on chamber has passed the life-span test of 40000 seconds at a throat temperature of 1400 degrees or more.Therefore,the protection of MoSi 2 coating shows a great significance for improving engine performance and extending its life for spacecraft.However,the thickness and uniformity of the molybdenum layer made in the process of vacuum ion plating has been found as a key in the preparation.And then,the compatibility of the infiltration siliconization to the molybdenum layer thickness also has an important impact on the reliability of the bipropellant engine.Once the molybdenum layer remains unexpectedly in the coating,it could seriously cause structural abnormalities of coating diffusion layer and significantly affect coating quality.In this case,the combination of unbalanced thermal stability and weakened binding force may result in penetrating cracks in the firing process,furthermore leading to a serious consequence in the working operation.In particular,this problem is more likely to occur in the area downstream of the throat.Because the temperature gradient here is large and the oxygen concentration is low,it tends to generate large temperature stress,leading to structural damage without enough self-healing.Therefore,it is proven to be important to strictly control the thickness of the molybdenum layer,properly prolong the infiltration time,and comparatively use the weighing inspection.Those methods aim to ensure that the MoSi 2 coating has a sufficient molybdenum to keep the enough thickness of the coating,and all molybdenum can simultaneously be silicified completely without residue.The quality of the coating preparation can be improved and the engine′s working reliability can be guaranteed in this way.