A.W. Mullendore

Thick boride coatings by chemical vapor deposition

In this paper we describe an experimental study of the chemical vapor deposition (CVD) of TiB2 by the hydrogen reduction of TiCl4 and BCl3 with the purpose of obtaining very thick (more than 100 μm) and uniform coatings. The optimum deposition conditions were as follows: temperature, 900–950 °C; source gas ratios, [Ti]/[B] = 12 and [H]/[Cl] = 61. With these conditions, deposition rates greater than 25 μm h-1 were obtained. The composition was very uniform with an excess of boron over stoichiometry (probably in the form of free boron). A small amount of chlorine remained incorporated in the deposit probably as TiCl2 (0.05 wt.% at 950 °C). The coatings were very hard (about 3700 kgf mm-2) and hardness was uniform through the thickness. With careful control of the deposition parameters, the CVD of uniform coatings with thickness of 1 mm or more appears feasible.

The development and laboratory testing of low Z refractory coatings for fusion reactor limiters

Abstract A materials development program for low Z ( Z ) coated tokamak limiters is described. A variety of refractory coatings applied by chemical vapor deposition, plasma spraying, and chemical conversion to graphite and copper substrates have been evaluated. The results of laboratory testing for low energy hydrogen ion erosion, arc erosion, and thermal fatigue by pulsed electron beam heating are reported. The 250-V hydrogen ion erosion data, analysed in terms of the effect on plasma radiation losses from resultant contaminants, indicates near equivalence of the very low Z (B and B4C, for example) and the moderate Z (e.g. TiB2) coatings. Thermal fatigue testing has identified several candidate materials which can survive 1000, 3 kJ/cm2 pulses of electron beam heating. The development of prototype limiters of TiB2, TiC, and B coated graphite for testing in the ORNL ISX-B tokamak is described.

Low Z coatings for fusion reactor applications

Abstract A large number of potential coating and substrate materials for tokamak fusion reactor components have been evaluated as to their erosion rates under low energy (250–1000 eV) hydrogen ion bombardment. For further testing, promising candidate coating materials have been deposited by chemical vapor deposition (TiB 2 , TiC and B 4 C) and plasma spraying (Be, TiB 2 , TiC and VBe 12 ) onto metal and graphite substrates. The adherence of the coatings has been evaluated in pulsed electron beam heating experiments. A number of coating-substrate combinations have survived 30–50 one second pulses of heating at power densities of 2 kW/cm without apparent damage. Others fail by thermal stress induced spall, coating-substrate alloying reactions and/or substrate melt.

Thermal-shock- and fatigue-resistant coatings for magnetically confined fusion environments☆

Abstract Candidate coating systems of materials of low atomic number Z under development for fusion reactor applications were exposed to a maximum of 1000 cycles of electron beam pulse heating at 2 kW cm-2 for 1.5 s. The coating/substrate systems that we investigated include chemically vapor-deposited TiB2/graphite, chemically vapor-deposited TiC/graphite, chemically vapor-deposited B/graphite, plasma-sprayed TiB2/Cu, clad V/Cu, clad Ti/Cu and clad plus borided VB2/V/Cu. All the thin coatings obtained by chemical vapor deposition (CVD) survived the testing as did the plasma-sprayed TiB2/Cu. Coatings of CVD TiB2 and CVD TiC on Poco graphite showed fatigue cracking which may have propagated into the graphite substrates. Some of the clad materials showed failure due to coating melt and deformation and separation due to thermal ratcheting.

The chemical vapor deposition of boron at low temperatures

Abstract In this study we investigated the formation of boron coatings obtained from the dissociation of diborane as a function of temperature, pressure and degree of ionization. At a pressure of 1 atm, pure boron was formed only at deposition temperatures above 500°C; below this temperature, partial dissociation occurred and the deposited boron was combined with boron polymers. To remove these polymers (as they were formed), it was necessary to operate at low pressures (approximately 50 mTorr), thus allowing boron coatings to be deposited at temperatures as low as 350°C. By activating the diborane in an r.f. field, it was possible to deposit boron at temperatures as low as 150°C. In all cases the deposited boron was very dense, uniform and amorphous.

Thermal-fatigue testing of coatings for fusion-reactor applications

Abstract Thermal fatigue testing was performed on eight coated or clad materials which have potential application as limiters in pulsed tokamak fusion devices. They are (1) chemically vapor-deposited coatings of TiC, TiB 2 and boron on graphite, (2) plasma-sprayed TiB 2 on copper, (3) a chemical conversion coating of VB 2 on vanadium-clad copper, (4) titanium-clad copper and (5) vanadium-clad copper. Testing consisted of up to 1000 cycles of electron beam heating for 1.5 s at beam power densities of 1 and 2 kW cm −2 . Three materials, chemically vapor-deposited TiC and TiB 2 on graphite, and plasma-sprayed TiB 2 on copper, survived the 1000 cycle 2 kW cm −2 test with slight but acceptable damage. The most notable test failure was VB 2 on vanadium-clad copper which deformed severely by a thermal ratcheting mechanism and displayed subsurface melting.

Testing of low-Z-coated limiters in tokamak fusion devices☆

Extensive testing on a laboratory scale has been used to select those coatings most suitable for this environment. From this testing which included pulsed electron beam heating, low energy ion bombardment and arcing, chemical vapor deposited coating of TiB/sub 2/ and TiC on Poco graphite substrates have been selected and tested as limiters in ISX. Both limiter materials gave clean, stable, reproducible tokamak discharges the first day of operation. After one weeks exposure, the TiC limiter showed only superficial damage with no coating failure. The TiB/sub 2/ limiter had some small areas of coating failure. TiC coated graphite limiters have also been briefly tested in the tokamaks Alcator and PDX with favorable results.

Wear resistance of ion-implanted, chemically vapor-deposited nickel die inserts

Abstract High precision dies can be fabricated to a near-net shape by nickel vapor forming; however, die wear severely limits the precision of the product when these nickel dies are used in the molding of glass-filled epoxy parts. Ion implantation of N + into die surfaces improved the wear resistance of the dies and decreased the friction coefficient, as determined by pin-on-disc testing; the pins were made from glass-filled epoxy. Transmission electron microscopy showed that the N + implantation treatment transformed the surfaces of the dies to Ni 3 N; the improved wear resistance and the decrease in the friction coefficient are attributed to this hard interstitial compound. In contrast, the dual-ion implantation of Ti + and C + into die surfaces produced amorphous surface layers but did not modify either the friction coefficient or the wear resistance significantly. Friction and wear results are shown to be influenced by sliding velocity. The wear process is a combination of adhesion and abrasion.

Infrared thermography of a pump limiter on Macrotor

Infrared thermography in the 2–5 μm wavelength range has been used to observe the surface heating of a pump limiter module in the Macrotor tokamak. Modest temperature rises are attributed to energy deposition by particle fluxes during discharges. Comparison of measured and calculated values indicated that heat loads were determined by module geometry, plasma position, and plasma scrape-off distance. In addition to the gradual, long-term heating of the entire module, rapid (on a time scale < 10−3 s) surface heating and cooling, associated with disruptions, were observed. The time scales were consistent with energy deposition for short times and subsequent thermal conduction to the remainder of the module. As much as 10% of the total plasma energy was deposited on the module during these discharges, with up to approximately one-quarter of the energy deposition occurring during several 100 μs disruptions. Infrared thermography was found to be a valuable real-time diagnostic tool for studying the interaction of the plasma with an in-vessel component.

Thermal fatigue of a TiB2 coated, long pulse, high heat flux component ☆

A low Z plasma sprayed, refractory coating has been successfully applied to a water cooled copper target. The coating survived the severe thermal cycling without debonding. The copper substrate, though substantially cracked, also did not fail. These results suggest that it will be possible to engineer reliable low Z high heat flux components for use in long pulse fusion reactors.