Abstract:

Ni-P electroless coatings were prepared in nickel sulfate solution at different pH values of 4.5，5.5 and 6.5 with the purpose of determining the influence of pH value on microstructure，internal stress statue and thermal stability of the coatings.The laser curvature（LC）method was used to measure the residual stress level in the coatings.Inductively coupled plasma massive spectrometer（ICPM）was used to analyze the P content in the as-deposited Ni-P coatings.Scanning electronic microscopy（SEM）and transmission electronic microscopy with energy dispersive spectrum（TEM/EDS）were used to examine the surface morphology and internal phase structure of the coatings，respectively.Differential scanning calorimeter（DSC）was used to analyze the phase transformation and thermal stability of the coatings at a high temperature.The results showed that the Ni-P coating prepared at pH 4.5 had well-developed amorphous structure，the Ni-P prepared at pH 6.5 had thorough nano-crystal structure，and the Ni-P prepared at pH 5.5 had a mixed structure with middle level of P content and tensile level.The Ni-P prepared at pH 5.5 experienced obvious cracking even after 250℃ intentional heat treatment，and TEM result revealed the micro-cracking originated from the Ni/Ni-P interface.On the contrary，the relatively higher stability of Ni-P coatings prepared at pH 4.5 and 6.5 was ascribed to the former’s lower tensile stress level and latter’s much finer grain size.Besides，the diminished local stress concentration in the nano-structure and the inverse Hall-Petch effect might also contribute to the integrity of Ni-P coating prepared at pH 6.5.

Key words:

非晶态, 应力, 开裂, 化学镀, 镍-磷

摘要：

在硫酸镍体系中，pH值为4.5、5.5和6.5的情况下分别制备出3种镍-磷化学镀层，目的在于研究pH值变化对镀层微观结构、内应力水平及热稳定性的影响。采用激光弯曲（LC）应力测量方法对3种镍-磷镀层的应力值进行测量;用电感耦合等离子质谱（ICPM）测量镀层中的磷含量;用扫描电镜（SEM）和配备能量散射谱的透射电镜（TEM/EDS）对镀层的表面形貌及内部相结构进行观测;用差热分析仪（DSC）对镍-磷镀层在高温下的相变规律及热稳定性进行研究。结果表明，pH值为4.5和6.5时制备的镍-磷镀层分别为完整的非晶态和纳米晶结构;pH值为5.5时制备的镍-磷镀层为部分非晶态与纳米晶的混合结构，并且该镀层具有介于前两种镀层之间的P含量及内应力水平。pH值为5.5时制备的镍-磷镀层在经历250℃热处理时发生明显的开裂，TEM测试表明微裂纹起源于Ni/Ni-P相界面;而pH值为4.5和6.5时镍-磷镀层所表现出的高热稳定性分别在于前者更低的拉应力水平及后者的充分的纳米晶结构。pH值为6.5时制备的镍-磷镀层虽然有最高的拉应力值，但其内部充分细小的纳米晶结构会有效避免局部应力集中的发生。此外镀层中可能存在的“反Hall-Petch效应”也是其耐热性能好的原因之一。

关键词:

非晶态, 应力, 开裂, 化学镀, 镍-磷