Karol Iždinský

Brazing Technology for Plasma Facing Components in Nuclear Fusion Applications Using Low and Graded CTE Interlayers

In Plasma Facing Components (PFCs) for nuclear fusion reactors, the protective material, carbon based or tungsten, has to be joined to the copper alloy heat sink for optimum heat transfer. High temperature vacuum brazing is a possible joining process as long as a proper interlayer is introduced to mitigate the residual stresses due to the mismatch of thermal expansion coefficient (CTE). Pure copper can act as plastic compliant layer, however for carbon based materials a proper structuring of the joining surface is necessary to meet the thermal fatigue lifetime requirements. In this work pure molybdenum and tungsten/copper Metal Matrix Composites (W-wires in Cu-matrix) interlayers have been studied as alternative to pure copper for carbon based protective materials in flat tile configuration. Finite element simulations of the brazing process have been performed to evaluate the expected residual stress reduction near the metal-carbon interface. In fact it has been demonstrated that stiff low CTE interlayers can shift the peak stresses from the weak carbon-metal interface to the strongest metal-metal one. Relevant samples have been manufactured and subjected to preliminary metallographic and thermal shock tests. Results obtained so far are encouraging and active cooled mock-ups are being prepared for high heat flux testing. Research work is in progress as regards monoblock configuration with both Wf/Cu MMC and graded Cu/W plasma sprayed and HIPped layers.

Methods of Increasing Thermal Conductivity of Plasma Sprayed Tungsten-Based Coatings

Tungsten is the main candidate material for the armor of plasma facing components for ITER and future fusion devices [1]. Plasma spraying is an alternative method for manufacturing tungsten-based coatings, including composites and graded layers, having a number of advantageous features [2]. On the other hand, the main limitation to application of these coatings on high heat flux components, is their low thermal conductivity, originating in the layered structure [3]. This paper is focused on four methods of improving the coatings’ thermal conductivity. First method consists in modification of the basic spraying parameters, which have a direct influence on the coating structure and therefore properties. The other three methods involve post-processing of the coatings: molten copper infiltration, laser remelting and densification by HIPping. The latter encompasses also tungsten-copper composites of various compositions. Experimental results, including structural and thermal characterization, are presented for each method. Finally, the applicability of these methods, from the point of view of manufacturing the plasma facing components, is discussed.

Al-Limestone and Al-Granite Composites Prepared by Cold Spraying

Two types of aluminium metal matrix composites (Al-MMCs) were prepared by cold spray process. The first Al-MMC was reinforced with granite rock and the second was reinforced with limestone rock particles. Al powder and rock powders (granite or limestone) were mixed to homogeneous mixtures and sprayed onto the Al substrate. The microstructure of as-sprayed composites was compared with microstructure of MMCs reinforced with commercially available Al2O3. The microstructures of Al-MMCs reinforced with granite and limestone were affected by hardness of rocks, particle sizes and compositions of the mixture. The coating is formed through high velocity impact of solid powders. The diameter of Al powder was about 30 μm and diameters of rock powders were from 5 to100 μm. Rock particles are distributed uniformly through the coating, maintaining their irregular morphologies. However, some large sized particles cracked and fragmented. The powder porosity was approximately 2.3 % for both types of Al-MMCs. Porous microstructure leads to lower critical velocity. The results indicate that, introducing irregular morphologies and/or pores into the feedstock. High quality metallic coatings can be more easily deposited by cold spray.

Ni-NiO Porous Preform with Controlled Porosity Using Al 2 O 3 Powder

The Ni-NiO skeleton seems to be a good candidate for various applications in industry such as corrosion-proof filters or components in refrigerating systems and as preforms for reactive infiltration with molten metals.The present work was focused on preparation of Ni-NiO composite with higher, controlled porosity. Sintering of pure Ni powder always leads to a substantial closed porosity in almost whole sample volume [1,2]. To eliminate this, we added Al2O3 particles with diameter of-32 +20 μm into the Ni powder (-75 +45 μm diameters) and sintered this loose powder mixture (Ni + 25 vol. % Al2O3) in air by progressive heating up to 800 °C followed by 2 hours isothermal exposure. As a control, pure Ni powder was sintered under the same conditions. Thermal oxidation of loose powder samples performed in alumina crucible indicates that the strongest oxidation occurred in the top part of sample, while the bottom part was the least oxidized. Therefore, it was necessary to run the thermal oxidation once more, but out of the crucible, to ensure the sufficient diffusion of oxygen to the whole volume of sample.

Microstructure and Erosion Resistance of Zirconium Diboride Ceramics Infiltrated by Pure Copper

Cu/ZrB2 composite was prepared by gas pressure infiltration of molten metal into ceramic preform. Microstructure and erosion resistance of composite was investigated. The microstructure was analysed by light microscopy and scanning electron microscopy. The chemical compositions were analysed using energy dispersive X-ray spectroscopy. Good penetration of copper along the grain boundaries of the 60% porosity sintered ceramics was analysed in the whole volume of composite. The interfacial morphology shows the regular interfaces without any macroscopic reactions [1]. Cu/ZrB2 composite was subjected to 60 spark discharges to investigate the ablation resistance. Linear dependence of the amount of loss material on the number of electrical discharge analytical cycles for Cu/ZrB2 composite was determined.

Microstructure and Thermal Expansion of Hybrid - Copper Alloy Composites Reinforced with both Tungsten and Carbon Fibres

Tungsten as refractory material and high thermal conductive carbon fibres are promising candidates for production of copper matrix composites for high temperature applications. Three types of rod-like samples were prepared by gas pressure infiltration of different carbon/tungsten fibre preforms with copper and/or copper alloy (Cu-0.5Cr) respectively. The fibres are aligned parallel to rod axis and were combined with the tungsten wire cloth. The microstructure of prepared hybrid composites was examined. The samples were thermally cycled 3 times up to 550 °C at a relatively high heating/cooling rate (10 K/min) to touch real condition in applications where high heat is formed during short time. The thermal expansion behaviour in radial direction was also analysed. Results show that a combination of both types of reinforcements in rod-shapes samples insures good protection against composite disintegration during high temperature thermal loading.

Thermal Cycling of Copper Based Composite Reinforced with High Modulus Carbon Fibres

The thermal expansion behaviour of Cu-1Cr/C composite subjected to 5 thermal cycles in the temperature range 30 - 1000 °C was investigated. The coefficients of thermal expansions (CTEs) as low as 0.7 x 10-6 K-1 in longitudinal and as large as 24.0 x 10-6 K-1 in transversal direction were obtained. Electron microscopy observations confirmed the high structural stability of the thermally cycled composite as no signs of disintegration were observed within the applied thermal cycling conditions.

Microstructure and Properties of Composites Prepared by Reactive Pressure Infiltration of Aluminium into Metal and Ceramic Powder Preforms

Nickel aluminides exhibit very attractive high temperature properties. However, due to high melting temperatures they are difficult to prepare. Gas pressure reactive infiltration is a relatively cheap technology that provides composites where nickel aluminides are formed due to mutual reaction between Ni powder and molten aluminium forced to penetrate into powder preform. The feasibility of this concept is demonstrated in this work. Ni powder and/or Ni+25 vol. % Al2O3 powder mixture, respectively, were mechanically pressed and then infiltrated with aluminium using 5 MPa argon gas pressure at the temperature of 750 °C for 120 s. Al/Al2O3 composite using loose alumina powder was prepared in similar manner for comparison. The microstructure of composites was observed by scanning electron microscopy and newly formed intermetallic phases were analysed by energy-dispersive X-ray spectroscopy. Relative elongations during additional thermal cycling up to 800 °C had been recorded. Composites were additionally characterized by hardness measurements.

The Effect of Ni Interlayers on Compaction of Mo/Mo Silicide Composites

Abstract: Reactions during compaction of Mo/Mo silicide wires with Ni interlayers are qualitatively assessed in this work. It appeared that due to extreme high temperature strength of MoSi2 hot pressing even at 1800°C/60 min/30 MPa in vacuum had not been sufficient to compact the Mo/Mo silicide wires in the absence of any additional interfacial layer. Therefore Ni had been chemically coated on the surface of Mo/Mo silicide wires that were subsequently compacted by hot pressing. Structural analysis revealed the reaction between Ni and MoSi2 resulting in the formation of ternary (MoNiSi) compounds. These established an interfacial bonding with minimal porosity.

Thermal Conductivity and Thermal Expansion of Copper Matrix Composites Reinforced with High Modulus C Fibres

Copper matrix composite with pure copper matrix reinforced with high modulus carbon fibres Thornel K 1100 was prepared by gas pressure infiltration technique. As-received composite was subjected to thermal expansion and thermal conductivity measurements in longitudinal and transversal directions. Large anisotropy of properties as well as surprisingly good structural stability has been observed. The mean coefficients of thermal expansion as low as 0.8 x 10-6 K-1 in longitudinal and as high as 23.5 x 10-6 K-1 in transversal directions were determined, the thermal conductivities as high as 650 Wm-1K-1 in longitudinal direction and as low as 60.7 Wm-1K-1in transversal directions were measured.