X.B. Zhou

Reactive wetting of liquid metals on ceramic substrates

This paper deals with a well known puzzling observation that sometimes the wetting is improved by a chemical reaction between a liquid and a solid substrate and sometimes just the opposite effect takes place. Here, contact angles of liquid Al on SiO2 and liquid Ti on Al2O3 have been measured. The surface and interface structures have been explored by scanning electron microscopy and energy dispersive X-ray spectrometry. According to the experimental observations, it turns out that the volume change of ceramic substrates during reaction plays a key role in the effect of chemical reaction on wetting. If the volume of ceramic substrate decreases after reaction, the wettability is not improved by the chemical reaction. This is the case of liquid Al wetting on SiO2. If the volume of ceramic substrate increases after reaction, the wettability is improved by the chemical reaction. This is the case of liquid Ti wetting on Al2O3. Besides our experimental observations, results from literature have been reviewed as well, which are in good agreement with the predictions based on the volume change criterion of ceramic substrate proposed in this paper.

INFLUENCE OF SURFACE-ROUGHNESS ON THE WETTING ANGLE

In this paper the influence of surface roughness on contact angles in the system of liquid Al wetting solid surfaces of Al2O3 has been studied. It was observed that contact angles of liquid Al vary significantly on different rough surfaces of Al2O3 A model is proposed to correlate contact angles with conventional roughness measurements and wavelengths by assuming a cosine profile of rough grooves with a Gaussian distribution of amplitudes. In comparison with the experimental results, the model provides a good estimate for describing the influence of surface roughness on contact angles of liquid Al on Al2O3.

Metal-Ceramic Interfaces in Laser Coated Aluminium Alloys

A novel process was developed to firmly coat an aluminum alloy, Al6061, with [alpha]-Al[sub 2]O[sub 3] by means of laser processing. In this approach a mixture of SiO[sub 2] and Al powder was used to inject in the laser melted surface of aluminum. A reaction product [alpha]-Al[sub 2]O[sub 3] layer of a thickness of 100 [mu]m was created which was well bonded to the aluminum surface. Various interfaces, Al/[alpha]-Al[sub 2]O[sub 3], Al/mullite and [alpha]-Al[sub 2]O[sub 3]/mullite, were studied by conventional transmission electron microscopy (CTEM) and high resolution electron microscope (HREM). It turns out that the presence of the Al/mullite interface may be essential to form a well bonded oxide layer and the high Si-content [alpha]-Al[sub 2]O[sub 3] intermediate layer may be wetted better by liquid Al. Investigations of the interface structures and wetting phenomena during laser processing are presented and a simple correlation between wetting phenomena and interface strength is derived.

Microstructure and tribological behavior of tungsten-containing diamondlike carbon coated rubbers

Tungsten-containing diamondlike carbon (W-DLC) coatings have been deposited on FKM (fluorocarbon), ACM (acrylate), and HNBR (hydrogenated nitrile butadiene) rubbers via unbalanced magnetron reactive sputtering from a WC target in C2H2/Ar plasma. The surface morphology and fracture cross sections of coated rubbers have been scrutinized by high resolution scanning electron microscopy (SEM). The random crack networks formed due to the large difference in the coefficients of thermal expansion break down the W-DLC coatings into segments of a couple of hundred micrometers in size, facilitating good flexibility if the interfacial adhesion between the coating and a rubber substrate is strong enough. The size and density of growth defects in the W-DLC coatings strongly depend on the surface roughness of the rubber sheets. The tribological behavior of uncoated and coated rubbers has been investigated with ball-on-disk tribotest under dry sliding condition against a 6 mm 100Cr6 ball. Uncoated rubbers exhibited a ver...

A reaction coating on aluminium alloys by laser processing

An aluminium oxide layer of 100 µm in thickness has been successfully coated on aluminium alloy 6061 and pure aluminium using a powder mixture of silicon oxide and aluminium by laser processing. A strong Al/Al2O3 interface was formed. The exothermic chemical reaction between SiO2 and Al may promote the metal/oxide wetting and the formation of Al2O3 layer. This new approach of ceramic coating on metals using a chemical reaction of other ceramics with metals may be applied to other systems.

SPINEL METAL INTERFACES IN LASER COATED STEELS - A TRANSMISSION ELECTRON-MICROSCOPY STUDY

This paper reports on coating a Duplex steel SAF 2205 and stainless steel 304 by bringing a mixture of Cr2O3 and Fe powder into a laser beam. Transmission electron microscopy reveals that in the case of proper bonding between substrate and coating a spinel structure around the composition FeCr2O4 could always be found near the interface. The Duplex steel transforms into a b.c.c. structure, whereas SS304 maintains its f.c.c. structure after laser treatment. Particles with a spinel structure have been observed in both the b.c.c. and the f.c.c. substrate. Crystallographic orientation relationships have been identified for the interfaces of the spinel structure both with the b.c.c. and f.c.c. matrix using electron diffraction. Planar faults have been observed in the spinel particles. No cracks have been found at the spinel/b.c.c. interface, whereas some cracks were detected at the spinel/f.c.c. interface in case there did not exist a crystallographic orientation relationship.

Structure-property relationship of metal-ceramic interfaces

The main emphasis of this book lies on the structure-property relationship, so as to bridge the length-scale gap involved in the study of materials stability. This paper focuses on several aspects within this framework by looking at the detailed atomic structure on a nanometer scale. Next, by using this information a transition is made via a mesoscopic length scale to explain the mechanical properties of metal-ceramic interfaces at a macroscopic scale. The basic philosophy is that depending on the material property one likes to investigate, it is necessary to consider the appropriate length scale where the relevant processes take place. The challenging part of this mesoscopic approach is of course to define the physical basis at the various length scales and the corresponding transitions involved. For that reason basically two case studies will be worked out. The first one is dealing with the atomic description of metal-ceramic interfaces using high resolution transmission electron microscopy as the main tool. The question to be answered is whether this image information can be interpreted in terms of the atomic structure and whether the latter will provide some information of the bond-strength between two dissimilar materials like a metal and a ceramic material. In order to make the transition from this nanoscale regime to a mesoscopic scale an anisotropic linear elasticity description will be employed as well. As a matter of course the way the material is processed is usually crucial for the mechanical property. For ultra high resolution transmission electron microscopy it is of great help to have well defined samples and for that reason the research on a nanoscale is carried out on internally oxidized metals with reasonably well defined ceramic precipitates.

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.

Al/γ-Al2O3 Interface in Laser Coated Aluminium Alloys

Laser coating with a hard layer on top of ahnninium alloys may significantly improve the wear resistance. However the selection of a particular coating material on ahuninium alloys is rather subtle. For instance the solubility of metals in aluminium is sometimes limited [ 11, e.g. a homogeneous layer of Ni, Mn and Co could not be formed by laser pmcess& [2]. In addition as the wetting of aluminium on A&O, is poor, a well bonded layer of A&O, could be hardly formed [3]. The large di&rences of melting points and thermal expansion coefbcients between A&OS and Al are also troublesome in the coating processing. In order to overcome these drawbacks, we have developed a laser coating process by means of a chemical reaction, where a reactive powder of SiO, [3] or Cr,O, [4] has been applied on the surface of aluminium alloys. A layer of reaction product Alz03 with 100 pm in thickness has been fmned on alum&m alloys. Here we report a coating using Cr,O, powders. According to the X-ray difli-action and energy dispersive spectrometry (SDS ) results in the following exothermic chemical reaction takes place Cr,O, + Al = Al,O, + Cr during the laser coating. Assuming that the reaction is pmcessed at 9OO”C, then the fbze energy and the exothermic heat of the reaction can be calculated as -455 kJ/mole and 538 kJ/mole, respectively [4]. As it was pointed out earlier these negative fiee energy and exothermic heat are essential to the formation of a thick and well bonded coating on ahmlmium alloys [3][4]. It is known that the interface structure may affect the interface properties, e.g. wetting and bonding, and consequently the quality of the coating in practical applications. Therefore, here we concentrate on the interface structure that has been studied by transmission electron microscopy (TEM) and high resolution transmission eleotron microscopy (HREM).

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.