Catalytic ceramic conversion of Zr702 alloy surfaces for combating wear and corrosion

Xiong, Xinhe (2023). Catalytic ceramic conversion of Zr702 alloy surfaces for combating wear and corrosion. University of Birmingham. Ph.D.

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Abstract

Due to their attractive properties, Zr and its alloys are widely used in some industrial sectors including nuclear equipment and medical devices. However, their low hardness, high friction and poor wear resistance have prohibited their tribological and tribo-corrosion applications. Recently developed novel catalytic ceramic conversion treatment (CCCT or C3T) has been shown to successfully improve the tribological and tribo-corrosion properties of titanium alloys with significantly reduced treatment time and energy consumption, while effectively increasing the production efficiency as compared to the conventional ceramic conversion treatment (C2T). However, the feasibility of applying the C3T technique for improving the tribological and tribo- corrosion properties of Zr and its alloys has not been investigated. Therefore, this research aims to improve the mechanical, corrosion and wear properties of commercial grade Zr702 alloy by developing a new C3T technique tailored for Zr alloys.

In this research, the C3T method has been investigated for Zr702 alloy with pre- deposited films of Ag, V, Au, Pd, Ag/V and Ag/Pd in the temperatures range of 450 - 600 °C, with treatment times between 5 minutes and 60 hours. The surface morphology and the microstructure of the treated samples were fully characterised using optical microscopy, SEM/EDX, XRD, TEM and GDOES; the mechanical, corrosion and tribological properties were evaluated using nano-& micro hardness indentation, electrochemical corrosion tests, and tribo-tests, respectively.

The results have shown that the pre-deposited catalytic films can effectively speed up the ceramic conversion process to produce a significantly thicker zirconium oxide layer with finer columnar structure as compared to the zirconium oxide layer formed using the conventional C2T process. For the four catalytic elements investigated in this research, under 600°C for 10-hour treatment, the catalytic effects in terms of increasing the surface oxide layer thickness were observed in the following order of Ag>V>Pd>Au. Although the specific catalytic mechanism is element dependent, enhanced oxygen absorption and inward diffusion promoted by the refined columnar structure of the oxide layers are common features for all the C3T treatments. The optimal C3T conditions were found to be 550-600°C for 40-60 hours, especially for V and Au pre-deposited C3T. All C3T treated samples were shown to have improved corrosion properties in comparison with C2T treated samples. This was found to be associated with thick, dense and uniform oxide layer formed during the C3T, which can prevent the direct contact of aggressive ions attacking the metal surface. The Au pre-deposited C3T sample demonstrated the best anti-corrosion performance and was shown to reduce the corrosion rate to only about 2% and 3% that of the untreated and C2T samples respectively. The surface hardness was measured to increase from 150 HV for the untreated Zr702 to 1400 HV for the V pre-deposited C3T sample. The C3T was shown to reduce the coefficient of friction from 0.65 to less than 0.25 and provide two orders of magnitude reduction in the wear factor due to the formation of thick and dense oxide layers.

It is expected that the developed C3T technique can pave the way towards wider applications of Zr and its alloys especially under challenging tribological and tribo- corrosion environments.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):