Characteristics of polysilazane compound and its application as coating for carbon material

Abstract

Organosilicon polymer, a ceramic precursor, has high formability and applicability. Thermal decomposition of the polymer produces a ceramic, which is mainly composed of silicon, carbon and nitrogen, and has excellent heat resistance and mechanical properties. Studies on fibers and composite materials of the Si–C system [1–3] and the Si–N system [4–7] obtained from organosilicon polymer are under way. In this study, we focused on polysilazane whose main structural framework consists of Si and N [8, 9]. Isocyanate-modified polysilazane is cured easily when it is heated with free radicals such as those produced by decomposition of dicumyl peroxide. This polysilazane, with a vinyl group, is liquid and stable, but difficult to hydrolyze. This liquid polysilazane is used as the starting material of coating materials, fibers and composite materials. In this study, we attempted to clarify the characteristics of liquid polysilazane and to apply it as an anti-oxidation layer of carbon materials. In the experiment, we used CERASET SN-L (LANXIDE, hereafter called CSN), which is a colorless, transparent, liquid polysilazane compound. Fig. 1 shows the estimated structure of CSN (according to LANXIDE report). We added 0.1 wt% of dicumyl peroxide to CSN as a polymerization initiator. Then, we measured the viscosity, refractive index and IR spectrum of this CSN solution. Next, hardness (Vickers hardness), electric resistance (insulating resistance) and bulk density of the cured CSN, which was produced after heating CSN at 150 ◦C, were measured. We performed thermogravimetry of the cured CSN at temperatures of up to 1000 ◦C at a heating rate of 10 ◦C/min in air. Then we measured the IR spectra of cured CSNs processed at 150 ◦C, 300 ◦C and 600 ◦C. In order to reveal the structural changes taking place in CSN at high temperatures, we subjected cured CSNs heat-treated at 1000 ◦C, 1200 ◦C, 1400 ◦C, 1600 ◦C and 1800 ◦C in vacuum to X-ray diffraction analysis. We used an artificial graphite plate (Hitachi Kasei, PD610, 25 × 25 × 5 mm) as a carbon substrate. Prior to being coated, the plate was cleaned with water and acetone. The procedure for coating treatment is as follows: the artificial graphite plate was immersed into a solution of CSN in toluene (10 vol%) for five minutes. After drying in air, the plate was heated for 1 h at a rate of 50 ◦C/h to 150 ◦C. Then, the plate was heated at a rate of 120 ◦C/h to 1000 ◦C in a tube furnace under argon atmosphere to fabricate a ceramic-coated artificial graphite plate. We obtained the amount of the anti-oxidation layer on the artificial graphite plate by calculating the weight gain of the plate. The thickness of the plate before and after coating treatment was measured by a micrometer to obtain the thickness of the film coating. We also fabricated two groups of samples, one subjected to three coating and heat treatment cycles, the other subjected to five. The obtained coated artificial graphite plate was heated in air at a rate of 120 ◦C/h to 800 ◦C. To study oxidation resistance, the ratio of weight loss of the plate was calculated by the change in weight after heating at 800 ◦C. Then, we observed the surface of the samples before and after oxidation resistance tests using a scanning electron microscope (SEM). The viscosity of CSN was 42.5 mPas (42.5 cP) and its refractive index, ND, was 1.4875. The cured CSN subjected to heat treatment and hardening at 150 ◦C was white and transparent, and had a homogenous structure with neither voids nor cracks. Vickers hardness of the cured CSN (heat treatment temperature, hereafter called HTT, 150 ◦C), Hv, was 49.13, electric resistance (insulating resistance) was 0.5–2.0 × 108 · m and bulk density was 1062 kg/m3. The absorption peaks of the N–H bond (3390 cm−1), the C–H bond due to methyl groups (1410 and 2980 cm−1), the Si–H bond (2160 cm−1) and the Si–N bond (950 cm−1), which was the principal chain of CSN, were observed from the IR spectrum of CSN (Fig. 2-1). The absorption peaks of the methyl group (1410 and 2980 cm−1) and the vinyl group (1600 and 2900 cm−1), which constitute the side chain, were also confirmed. The IR spectra of CSNs subjected to hardening after heat treatment at 150 ◦C in air (Fig. 2-2), and subjected to heat treatment at 300 ◦C (Fig. 2-3) and 600 ◦C (Fig. 2-4) after heat treatment at 150 ◦C, are likewise shown. For the cured CSN subjected to a HTT of 150 ◦C in air, the intensities of the absorption peaks due to the N–H bond, the C–H bond and the vinyl group were markedly decreased, while the peak of the Si–N bond at HTT of 300 ◦C was hardly observed.