学 位 記 番 号 シス博 第

氏 名 金

勇

所 属 システムデザイン研究科 システムデザイン専攻

学 位 の 種 類 博士（工学）

学 位 記 番 号 シス博 第

号 学位授与の日付 平成

年

月

日

課程・論文の別 学位規則第４条第１項該当

学 位 論 文 題 名 D e p o s i t i o n o f B o r o n N i t r i d e F i l m s b y H o t F i l a m e n t C V D a n d E v a l u a t i o n o f i t s T r i b o l o g i c a l P r o p e r t i e s (熱 フ ィ ラ メ ン ト C V D 法 に よ る 窒 化 ホ ウ 素 膜 の 合 成 と ト

ラ イ ボ ロ ジ ー 特 性 評 価 ) 論 文 審 査 委 員 主査 教 授 楊 明

委員 准教授 金子 新 委員 准教授 菅原 宏治

委員 教 授 相澤 龍彦(芝浦工業大学)

【論文の内容の要旨】

Microforming has received good attention in the manufacture of microparts due to its high production rates, excellent mechanical properties of products and near net shape production. Titanium is widely used in the production of medical components and as biomaterial for implants because of its superior biocompatibility, corrosion resistance and specific strength. Thus microforming of titanium is expected to be widely applied to produce micro medical components. However, the galling behavior of titanium is often occurred during the microforming process since pure titanium has low abrasion resistance and high reactivity. The galling behavior would reduce the lifetime of tools and influence the surface quality of products. Futhermore, no lubricant is used in microforming process since it has low effect with miniaturization and the products are difficult to be cleaned. Thus hard coatings with low friction are considered to be used for the forming tools to prevent the galling behavior.

Cubic boron nitride (cBN) film is a good candidate of these hard coatings. It has similar mechanical properties as diamond with high hardness and high wear resistance.

Moreover, it has high temperature stability and chemical inertness which are superior

to diamond. Normally cBN films are prepared by physical vapor deposition (PVD) or

plasma enhanced chemical vapor deposition (PECVD) using highly-energetic ions/plasma

bombardment during the deposition process. This would causes several issues in the cBN films, such as big residual stress which leads films to crack and delaminate; low adhesion strength between the films and substrate and the limitation thickness less than 1μm. Moreover, the friction coefficient of cBN films is high. These issues of cBN films limit its application as anti-wear and protective coatings for metal forming tools. On the other hand, hexagonal boron nitride (hBN) has low friction coefficient which is often called ‘white graphite’, the additional of hBN phase in the cBN films could reduce its friction coefficient and improve the tribological properties of the films. However, the process for deposition of compound BN films has not been investigated.

The purpose of this study is to deposit hBN and cBN containing compound BN films for the application of metal forming tools. This compound BN films could have the outstanding tribological and wear properties by optimizing the ratio of hBN to cBN concentrations. A hot filament chemical vapor deposition (HFCVD) based system is developed for deposition of this compound BN films. A bias voltage is additionally applied between the substrate and filament in this system, the distance between the filament and substrate can be adjusted. In comparison with PECVD process which using plasma to activate precursor for deposition of films, thermionic emission from hot filament is used in this process. This thermionic emission current and energy can be enhanced by filament temperature, bias and filament distance to achieve the threshold for cBN films nucleation. Compound BN films with different ratio of cBN to hBN concentrations are deposited by changing several deposition parameters in this process. The influence of deposition parameters on the cBN films nucleation and formation is investigated. A parameter window based on the experimental results is proposed to predict the ratio of cBN concentration in the compound BN films. Moreover, the tribological properties and interfacial reaction between the compound BN films and pure titanium is carried out by a ball-on-disk (BOD) test to verify the possibility of the compound BN films applied for metal forming tools. The introduction and conclusions of this study are divided into 6 chapters which are shown as follows:

Chapter 1. The background of microforming of titanium and common deposition methods of cBN films are introduced. Based on the issues of galling behavior of titanium and drawbacks of deposited cBN films, the motivation and objectives of this study are proposed.

Chapter 2. A Hot Filament Chemical Vapor Deposition (HFCVD) based system is

developed. In this improved HFCVD system, a bias is additionally applied between the

substrate and filament, and the distance between filament and substrate can be

adjusted. Thermionic emission from hot filament is used for deposition of cBN films which is mainly influenced by filament temperature. A novel precursor both containing B and N is used in this study. To verify the possibility of this precursor and improved HFCVD system used for deposition of cBN films, decomposition rate of precursor is studied and controlled by varying several deposition parameters. The results show hBN films are obtained by optimizing the ratio of precursor to ammonia gas flow rate and with high filament temperature.

Chapter 3.In order to deposit cBN films by this improved HFCVD system, deposition energy is enhanced by changing the bias and filament distance. The influence of these deposition parameters on the cBN films nucleation and formation is investigated systematically. Compound BN films with different ratios of cBN to hBN concentrations are obtained by optimizing the bias and filament distance. A parameter window for the cBN films nucleation in this process is determined.

Chapter 4. Based on the nucleation models of cBN films deposited by PVD and PECVD process, a possible nucleation model for cBN films deposited by this improved HFCVD system is proposed. The available data from the experimental results in this study are collected and suggest that a combination effect of ion energy, ion flux and catalytic effect from hot filament may influence the cBN nucleation. Moreover, in order to predict the cBN ratio in the compound BN films deposited by this system, a parameter window of cBN ratios in the compound BN films related to bias and filament distance is proposed.

Chapter 5. A Ball-On-Disk test (BOD) is carried out to investigate the tribological properties and interfacial reaction between the compound BN films and pure titanium. The results show the lower ratio of cBN concentration in the compound BN film has lower friction coefficient and lower wear resistance. Higher ratio of cBN concentrations could improve its wear resistance. Due to the large surface roughness, friction coefficient of the compound BN films is high. Optimization of deposition conditions are needed to smooth the films surface. Interfacial reaction between the compound BN films and pure titanium is low which indicates the potential application of compound BN films being used as the protective coatings for metal forming of pure titanium.

Chapter 6. Summarizes and concludes the innovation and contribution of this study.

The defects and remaining problems are pointed out, and the future works are suggested.