偏压梯度TiAlN涂层对TC4钛合金振动与拉伸疲劳性能的影响与机理

Effect and Mechanism of Bias-graded TiAlN Coatings on Vibration and Tensile Fatigue Properties of TC4 Titanium Alloy

目的 探究偏压梯度TiAlN涂层对基体疲劳性能的影响规律和疲劳损伤机理。方法 利用磁过滤阴极真空弧技术和连续改变偏压的沉积工艺,在TC4钛合金表面沉积了偏压梯度TiAlN涂层,并采用扫描电镜、轮廓仪、纳米压痕和划痕仪表征测试了TiAlN涂层的微观结构和内应力、表面硬度、膜基结合力等基本力学性能。对TiAlN涂层试件的振动和拉伸疲劳性能分别进行了考核,通过观察试件疲劳断口形貌,探究了偏压梯度TiAlN涂层/基体的疲劳损伤机理。结果 TiAlN涂层中Al元素含量沿深度方向一直在降低,偏压工艺成功制备出梯度结构涂层。偏压梯度TiAlN涂层的内应力为压缩状态,数值为(2.66±0.23) GPa,显著低于对应恒压涂层(-200V)。偏压梯度TiAlN涂层试件平均振动强度和拉伸疲劳强度分别为370.90、377.90 MPa,前者相对于TC4基体提高了47.7%,后者几乎保持不变。结论 TiAlN涂层内部存在残余压应力,具有一定抗裂纹萌生能力,TC4钛合金表面制备偏压梯度TiAlN涂层后,两种受载类型下的疲劳裂纹源均位于涂层与基体界面处。振动受载时,涂层中梯度结构抑制了裂纹的扩展,疲劳强度提高;拉伸受载时,TiAlN涂层部分发生破碎,抑制裂纹萌生与促进裂纹扩展两种机制同时存在,疲劳强度几乎不变。

The erosion resistant coating on the surface of the blade material can improve the sand protection performance.However,the aero-engine compressor blades are subject to the airflow excitation force aroused during the working process,causing the resonance of the blade in the low-order mode.The blades rotate at high speed,bearing huge centrifugal force.After the anti-erosion coating is prepared on the surface of the blade material,it is prone to cause fatigue damage together with the substrate under the above two types of alternating load,influencing the fatigue performance of the substrate.To investigate the effect of the coating on the fatigue properties of the substrate and the fatigue damage mechanism,bias-graded TiAlN coatings was deposited on Ti6Al4V alloy substrates using filtered cathodic vacuum arc(FCVA) technology with the bias-graded deposition method,during which the negative bias was changed gradually from-50 V to-200 V.The microstructure of the bias-graded TiAlN coating was observed and examined with a scanning electron microscopy.The basic mechanical properties including the element distribution along the depth direction,internal stress,surface hardness and film-substrate bonding force were characterized by an energy dispersive spectroscopy,a profilometer,a nanoindentation and scratch meter,respectively.The vibration and tensile fatigue properties of the bias-graded TiAlN coating/substrate specimens were evaluated and the fatigue damage mechanism was analyzed by observing the fatigue fracture morphologies of the specimens.The results showed that the coating surface was dense and uniform with few droplets.The Al content along the depth direction of the TiAlN coating was decreased gradually,indicating the gradient structure coating was successfully prepared.The internal stress of the bias-graded TiAlN coating was(2.66±0.23) GPa,which was in a compressive state and significantly lower than that of the constant-bias coating.The inner defects created under the lower bias were more easily removed by the ad-atoms formed under the higher bias during the diffusion process,leading to the lower internal stress in the bias-graded TiAlN coating.The binding force of the bias-graded TiAlN coating was 44.03 N,which was much higher than that of the constant bias TiAlN(-200 V) coating.The gradient structure could reduce the stress concentration and the internal stress of the coating,improve the crack resistance of the coating under vertical load,thereby increasing the bonding force between the coating and the substrate.The average vibration strength and tensile fatigue strength of the TiAlN coating specimens were 370.90 MPa and 377.90 MPa,respectively.The former was increased by 47.7% compared with the TC4 substrate,and the latter was almost unchanged.The residual compressive stress existed in the TiAlN coating can resistant to crack initiation.After the bias-graded TiAlN coating is prepared on the surface of the TC4 specimen,the fatigue crack sources under the two types of loading are both located at the interface between the coating and the substrate.For the vibration loading,the gradient structure in the coating inhibits the growth of cracks,and thus the fatigue strength increases.While for the tensile loading,the TiAlN coating is partially broken,and the two mechanisms of inhibiting crack initiation and promoting crack growth exist simultaneously,thus the fatigue strength is almost unchanged.