Investigation On The Microstructure And Mechanical Properties For WC_xN_y And NbC_xN_y Thin Films

As a well-known refractory material, tungsten carbide films are widely used in the industrial applications because of their high hardness, high elastic modulus, wear resistance and chemical inertness. They have different structures including hexagonalα-WC, cubicβ-WC1-x with carbon-deficient and W2C which is considered as a compound in both hexagonal and orthorhombic forms. The properties of tungsten carbide films strongly depend on their phase structure. For example,β-WC1-x has a higher superconducting transition temperature than that ofβ-W2C orα-WC, andα-WC usually exhibits better wear resistance and higher hardness in comparison with W2C orβ-WC1-x. Hence, it is necessary to control the phase of tungsten carbide thin films for tailoring applications. Although the different phases have been found in tungsten carbide thin films prepared by physical vapor deposition (PVD) or chemical vapor deposition (CVD) techniques, the low-temperature synthesis of the single-phaseα-WC films with the excellent mechanical properties remains an open challenge, and the mechanisms on phase transition in tungsten carbide thin films have not yet been well understood. Recent investigations have showed that doping is an effective way to improve the structure and properties for thin films. However, so far, there is no any report on the influence of N doping on the phase transition for tungsten carbide thin films.Tungsten carbonitride (WCxNy) thin films can be widely used in the fields of aviation, electronics and machinery because of their excellent properties. In some of these applications, the mechanical properties for WCxNy thin films is an important issue for their reliable long-term performance. Therefore, it is necessary to investigate the effect of deposition conditions on the mechanical properties for WCxNy thin films. However so far, there is no any report on the mechanical properties of WCxNy.Recent studies show that there is the hardness enhancement phenomonen in both Nb-Si-N and Ti-C-N nanocomposite films. We expect that the similar phenomonen may occur in the Nb-C-N nanocomposite films. However, so far, no investigation concerning the microstructure and mechanical properties of Nb-C-N nanocomposite films has been performed.In this work, we prepare the N-doped tungsten carbide films with different N contents (CN) on Si(100) by DC reactive magnetron sputtering at 500℃and explore the effects of N-doping on the preferred orientation, phase transition, and mechanical properties for the obtained films. Then, we fabricate a series of WCxNy films by magnetron sputtering and investigate systematically the effect of substrate bias, gas flow ratio and substrate temperature on the microstructure and mechanical properties of films. Finally, we deposite the NbCxNy thin films by magnetron sputtering and study the effects of CH4 flow on the phase structure and mechanical properties of NbCxNy thin films. The conclusions are summarized as follows:I.The effect of N doping on the microstructure and mechanical properties of tungsten carbide (WC) thin-filmsWe find that N doping has a significant influence on the preferred orientation, phase structure and hardness for films.As the content of N increases from 0 at.% to 1.4 at.%, the texture changes from a strongβ-WC (200) to (111).. As the content of N increases from 1.4 at.% to 2.9 at.%, a phase transition fromβ-WC toα-WC.occurs. The hardness of thin film increases gradually with increaseing the content of N, which is mainly due to the phase transition fromβ-WC toα-WC in the film because hexagonalα-WC is harder thanβ-WC. This investigation suggests that nitrogen doping in the tungsten carbide film is favor of the formation of hexagonalα-WC phase, which provides a new way to fabricate hexagonalα-WC films with excellent mechanical properties under the low-temperature condition.II.The effects of deposition parameters on the microstructure and mechanical properties of WCxNy thin films(1) Substrate bias has almost no effect on the composition of WCxNy thin films. As the absolute value of bias voltage increases to 40V, the texture changes from a strongβ-WCxNy (200) to (111), and at the same time a phase transition fromβ- WCxNy toα- WCxNy takes place. When the absolute value of bias voltage increases to 120V, the hardness of films reaches a maximum value of 47.5Gpa.(2) As CH4 flow increases to 1.8sccm, the texture changes from a dominantβ-WC1-x (111) to (200), As CH4 flow is higher than 1.8sccm, a phase transition fromβ-WC1-x toα-WC occurrs. With increasing CH4 flow, the grain size gradually decreases and harderα-WC phase is formed at the expense ofβ-WC1-x, which promotes the hardness enhancement of thin films.(3) Lower substrate temperature has little effect on the microstructure of WCxNy thin films. when substrate temperature is increased to 700℃, a phase transition from β-W3C toβ-WC1-X takes place, and simultaneously the texture changes fromβ-W3C (200) to (210) and (211).III.The effects of CH4 flow on the microstructure and mechanical properties of NbCxNy thin filmsWith increasing CH4 flow from 0.5 to 1.8 sccm, a phase transition fromδ′-NbN toδ-NbN occurrs. As CH4 flow increases from 2.1 to 2.5 sccm, the preferred orientation changes fromδ-NbN (220) to (200). As CH4 flow increases from 0.3 to 1.0 sccm, the hardness of the films increases from 24.8 GPa to the maxium value of 38.5 GPa, which is due to the formation of the NbN/CNx nano-composite structures in the film. However, When CH4 flow increases from 1.0 to 2.5sccm, the hardness of the films gradually decreases from 38.5 to 20.1 GPa, which is due to the excess formation of amorphous phase such as a-C in the films.