Thermal shock testing of low-Z coatings for JT-60

Abstract

It is considered that low-Z coatings will be in use for the first wall of JT-60. However, if the plasma were in off-normal operation and plasma energy were locally deposited on the first wall, the first wall would suffer a severe thermal load. Therefore, it is very important to investigate the adhesion between coating and substrate under thermal load [ 1.2,3]. The samples in use for the thermal shock testing are TIC and TIN coatings on MO and Inconel625 produced by chemical vapor deposition (CVD) and physical vapor deposition (PVD) [4]. The thickness of all coatings is 20 pm and some of the TIC samples have an intermediate layer (TIN or Ti) between coating and substrate, whose thickness is less than 1 pm. The substrate temperature during coating is higher than 1000°C for CVD and is lower than 500°C for PVD. The dimension of the samples is 10 mm X 10 mm X 0.5 mm for CVD TiC and CVD TIN, and is 12 mm X 12 mm X 0.5 mm for PVD Tic. The thermal shock testing has been made by means of irradiation with a pulsed beam of neutral and ionic hydrogen from Ion Source Test Stand-2 for NBI of JT-60. The power density of the hydrogen beam is 2kW/cm’ which is measured by two types of calorimeters, and the average particle energy and density are estimated at 30 keV/H and 4 X 10” H/cm’. s. respectively [5,6]. The sample set-up is shown in fig. 1. The target which is indicated by hatching in fig. 1 is irradiated with a pulsed hydrogen beam. The irradiation has been made by varying the duration time of a pulsed beam until sample melting. In fig. 2(a)-(c) are shown the results of the thermal shock testing of TIC and TIN coatings on MO. In the case of CVD Tic, microcracks grow by 0.3 s irradiation and the substrate is melted by more than 0.35 s irradiation. In case of CVD TiN, the middle of the irradiated surface is discolored by more than 0.3 s irradiation, and it has been made clear from X-ray Micro Analysis (XMA) that nitrogen is evaporated much more than titanium. The substrate under CVD TiN is melted by 0.4 s irradiation. CVD TiC and TiN are not exfoliated from the substrates in the circumference of the melted area. In the case of PVD TIC, the coating is severely exfoliated from the substrate by more than 0.2 s irradiation and the thickness of the exfoliated film is 20 pm, which shows that the exfoliation occurs at the interface between coating and substrate. The substrate of PVD TIC is melted by more than 0.25 s irradiation. Although all samples have the same thickness for coating and substrate, duration time of the pulsed beam until substrate melting is strongly dependent on the coating material and method. It seems that the dependence can be explained by the existence of the interdiffused layer between coating and substrate. When the interdiffusion between TIC and MO occurs more severely than between TiN and MO, the interdiffused layer of PVD TiC (TiC/(Ti)/(Mo) will be thicker than CVD TIC (TiC/TiN/Mo) and CVD TIN (TiN/Mo). And, when the melting point of the interdiffused layer is lower than that of the substrate, the melting of the interdiffused layer will cause the melting of the substrate. In fact, it is reported that the melting point of MO including 3% TiC is 2200°C. [7] which is 400°C lower than the melting point of pure MO. In fig. 3 are shown a micrograph and XMA signal of CVD TIC on Inconel 625 after 0.2 s irradiation. The composition of the exfoliated area is only TIC and the thickness of exfoliated films is 5 pm which is 4 times thinner than of coating. Therefore, in