M. van den Burg

AL-SIC INTERFACE STRUCTURE STUDIED BY HREM

Abstract A comparison is made between the two well-known aluminium alloys 2014 and 6061, cold pressed from powder and subsequently extruded, with and without a reinforcement of 30 wt% α-SiC particulates. The former one has a strength of 550 MPa and a strain at fracture of 1% while the latter one has a more moderate strength of 415 MPa but a fracture strain of 4%. A high resolution electron microscopy study of the Al SiC interface indicates that proper wetting is achieved in both alloys. A preferred orientation relationship is observed in the 6061-SiC combination: (0001)SiC//{111}Al; 〈 2 1 ¯ 1 ¯ 〉 S i C / / 〈 110 〉 A l . When the (0001)SiC is not parallel to the interface, a stepped interface is observed with the 6061 alloy, with one part of the step parallel to the SiC basal plane and one type of the Al octahedral planes and the other part parallel to another type of Al octahedral planes. In the 2014 alloy the steps seem to be less pronounced probably due the processing route.

Mechanical performance of metal-ceramic interfaces produced by laser processing

This paper concentrates on the mechanical performance of various ceramic coatings of Cr2O3 on steel (SAF2205), as produced by CO2 laser processing. The thickness of the coating that can be applied by laser coating is limited to about 200 μm setting a limit to the maximum strain energy release rate that can be measured in a 4 point flexure test before severe yielding occurs. In addition, a network of cracks with spacings of the order of 200 μm was always present in the laser applied coating preventing steady state crack growth along the interface.It is concluded that a firmly bonded coating of Cr2O3 on steel could be produced by high power laser processing. The actual interface strength of a (Fe, Cr)-spinel applied to stainless steel by laser coating depends strongly on the composition of the substrate and coating materials. The energy release rate was extremely high and delamination occurred by fracture through the coating and partially along the interface, indicating that the interface strength is similar to or higher than the fracture strength of (Fe, Cr)-spinel.

METAL-CERAMIC INTERFACES PRODUCED BY LASER MELT INJECTION PROCESSING

Abstract This paper concentrates on the mechanical performance of various ceramic coatings of Cr2O3 on steel (SAF2205), as produced by CO2 laser processing. It is concluded that a firmly bonded coating of Cr2O3 on steel could be produced by high power laser processing. The actual interface strength of a (Fe,Cr)-spinel applied to stainless steel by laser coating depends strongly on the composition of the substrate and coating materials. The energy release rate was extremely high and delamination occurred by fracture through the coating and partially along the interface, indicating that the interface strength is similar to or higher than the fracture strength of (Fe,Cr)-spinel.

MARTENSITIC TRANSFORMATIONS IN LASER PROCESSED COATINGS

This paper concentrates on laser coating of Fe-22 wt% Cr and a duplex steel SAF2205 by injecting Cr2O3 powder into the melt pool. In particular the work focuses on the stabilization of high temperature distorted spinel phases due to the high quench rates involved as well as on the a quantitative crystallographic analysis of the resulting morphologies. The microstructure observed in TEM indicates that the material does not solidify in the distorted spinel structure. The presence of a small amount of cubic (Fe, Cr)-spinel suggests that the distorted spinel in fact might be nucleated from the cubic spinel phase. The plate like morphology of the distorted spinel phase in combination with the twinned internal structure of the plates put forward the idea that the transformation might be martensitic. Martensitic calculations executed with the lattice parameters of the cubic and distorted (Fe, Cr)-spinel phases are in excellent agreement with the experimental data confirming that the transformation might be martensitic indeed.

Metal-Ceramic Interfaces in Laser Coated Steels : A Transmission Electron Microscopy Study of a Mixture of Iron and Spinel Grains

This paper concentrates on laser coating of a Duplex steel SAF 2205 by bringing a mixture of Cr2O3 and Fe powder into a laser beam. After the laser treatment the Duplex steel transforms into a b.c.c. structure. Transmission electron microscopy observations indicate a proper bonding between substrate and coating consisting of a spinel structure around the composition FeCr2O4 near the interface. In addition, particles with a spinel structure have been observed in the b.c.c. substrate. Crystallographic orientation relationships have been identified for the interfaces of the spinel structure with the b.c.c. matrix using electron diffraction.

MICROSTRUCTURE OF CR2O3 COATINGS ON STEEL AND THE EFFECT OF SILICON

This paper concentrates on the microstructural features of steel containing 22 wt.% Cr, coated with Cr2O3 by laser processing. It turned out that after laser coating the Cr2O3 powder has completely transformed to Fe0.3Cr2.7O4 having the tetragonal distorted spinel structure. Dispersed in the coating are metallic particles with composition FeCr and a bcc structure. The phases in the coating can be explained from the Fe-Cr-O equilibrium phase diagram with the assumption that complete phase equilibrium is reached in the liquid state but not during solidification. The two equilibrium phases, L(0) and L(m), that exist in the molten state solidify as two independent liquids. Addition of Si to the ceramic material, either from the steel matrix or from the ceramic powder, results in a dendritic solidification structure of Cr3O4 dendrites and a Si-containing glassy phase. The dendrites are oriented vertically in the coating, resembling the columnar microstructure that is also observed in ZrO2 thermal barrier coatings. This structure contains fewer microcracks parallel to the interface resulting in a mechanically more stable ceramic coating.

STRUCTURE-PROPERTY RELATIONSHIP OF METAL-CERAMIC INTERFACES PRODUCED BY LASER PROCESSING

A novel process was developed to firmly coat an aluminium alloy, A16061, with α-A1 2 O 3 by means of laser processing. In this approach a mixture of SiO 2 and Al powder was used to inject in the laser melted surface of aluminium. A reaction product α-A1 2 O 3 layer of a thickness of 100 μm was created which was well bonded to the aluminium surface.Various interfaces, A1/α-A1 2 O 3 , Al/mullite and α-A1 2 O 3 /;mullite, were studied by conventional transmission electron microscopy (CTEM) and high resolution electron microscope (HREM). It turns out that the presence of the A1/;muilite interface may be essential to form a well bonded oxide layer and the high Si-content α-A1 2 O 3 intermediate layer may be wetted better by liquid Al.

Structure-Property Relationship of Ceramic Coatings on Metals Produced by Laser Processing

This paper concentrates on the mechanical performance of various ceramic coatings of Cr2 O3 on steel (SAF2205), as produced by CO2 laser processing.The thickness of the coating that can be applied by laser coating is limited to about 200 μm setting a limit to the maximum strain energy release rate that can be measured in a 4 point flexure test before severe yielding occurs. In addition, a network of cracks with spacings of the order of 200 μm was always present in the laser applied coating preventing steady state crack growth along the interface.