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Thermal Processing Effects on the Adhesive Strength of PS304 High Temperature Solid Lubricant Coatings

机译:热处理工艺对PS304高温固体润滑剂涂层粘结强度的影响

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摘要

In this paper the effects of post deposition heat treatments on the cohesive and adhesive strength properties of PS304, a plasma sprayed nickel-chrome based, high temperature solid lubricant coating deposited on stainless steel, are studied. Plasma spray deposited coating samples were exposed in air at temperatures from 432 to 650 C for up to 500 hr to promote residual stress relief, enhance particle to particle bonding and increase coating to substrate bond strength. Coating pull-off strength was measured using a commercial adhesion tester that utilizes 13 mm diameter aluminum pull studs attached to the coating surface with epoxy. Pull off force was automatically recorded and converted to coating pull off strength. As deposited coating samples were also tested as a baseline. The as-deposited (untreated) samples either delaminated at the coating-substrate interface or failed internally (cohesive failure) at about 17 MPa. Samples heat treated at temperatures above 540 C for 100 hr or at 600 C or above for more than 24 hr exhibited strengths above 31 MPa, nearly a two fold increase. Coating failure occurred inside the body of the coating (cohesive failure) for nearly all of the heat-treated samples and only occasionally at the coating substrate interface (adhesive failure). Metallographic analyses of heat-treated coatings indicate that the Nickel-Chromium binder in the PS304 appears to have segregated into two phases, a high nickel matrix phase and a high chromium precipitated phase. Analysis of the precipitates indicates the presence of silicon, a constituent of a flow enhancing additive in the commercial NiCr powder. The exact nature and structure of the precipitate phase is not known. This microstructural change is believed to be partially responsible for the coating strength increase. Diffusion bonding between particles may also be playing a role. Increasing the heat treatment temperature, exposure time or both accelerate the heat treatment process. Preliminary measurements indicate that the heat treatment also results in a one time, permanent coating thickness increase of about 3%. Based upon these results, the incorporation of a heat treatment prior to final finishing has been incorporated in the application process of this coating technology.
机译:本文研究了沉积后热处理对PS304的内聚力和粘合强度性能的影响,PS304是等离子喷涂镍铬基高温固体润滑剂涂层,沉积在不锈钢上。将等离子喷涂沉积的涂层样品在432至650 C的温度下暴露于空气中长达500小时,以促进残余应力释放,增强颗粒间的粘结并增加涂层对基材的粘结强度。使用商用粘附力测试仪测量涂层的剥离强度,该测试仪利用直径为13 mm的铝制拉钉通过环氧树脂附着在涂层表面上。自动记录剥离力,并转换为涂层剥离强度。作为沉积的涂层样品也作为基线进行测试。沉积的(未处理的)样品在涂层-基材界面处分层或在约17 MPa的压力下内部失效(内聚破坏)。在高于540°C的温度下进行100小时或在600°C或以上的温度下进行24小时以上热处理的样品显示出高于31 MPa的强度,几乎提高了两倍。对于几乎所有热处理的样品,涂层内部都会发生涂层失效(内聚失效),而涂层基体界面则偶尔发生涂层失效(内聚失效)。热处理涂层的金相分析表明,PS304中的镍铬粘合剂似乎已分离为两相,即高镍基体相和高铬沉淀相。对沉淀物的分析表明硅的存在,硅是商业NiCr粉末中流动增强添加剂的组成部分。沉淀相的确切性质和结构尚不清楚。据信这种微观结构变化是涂层强度增加的部分原因。颗粒之间的扩散键合也可能起作用。提高热处理温度,暴露时间或同时提高两者都可以加快热处理过程。初步测量表明,热处理还导致永久涂层厚度一次增加约3%。基于这些结果,在最终涂饰之前加入热处理已纳入该涂层技术的应用过程中。

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