Oxide and non oxide ceramics (Al2O3, SiC) were brazed to commercial steel with active filler alloys using a CO2-laser
(l = 10.64 μm). Two different laser intensity profiles were used for heating up the compound: A laser output beam
presenting a Gaussian profile and a homogenized, nearly top head profile were applied for joining the compounds in an
Argon stream.
The temperature distribution with and without the homogenizing optic was measured during the process and compared to
the results of a finite element model simulating the brazing process with the different laser intensity profiles. Polished
microsections were prepared for characterization of the different joints by scanning electron micrographs and EDXanalysis.
In order to evaluate the effects of the different laser intensity profiles on the compound, the shear strengths of
the braze-joints were determined. Additionally residual stresses which were caused by the gradient of thermal expansion
between ceramic and metal were determined by finite element modeling.
The microsections did not exhibit differences between the joints, which were brazed with different laser profiles.
However the shear tests proved, that an explicit increase of compound strength up to 34 MPa of the ceramic/metal joints
can be achieved with the top head profile, whereas the joints brazed with the Gaussian profile achieved only shear
strength values of 24 MPa. Finally tribological pin-on-disc tests proved the capability of the laser brazed joints with
regard to the application conditions.
With the help of a CO2-laser (λ = 10.64 μm) Silicon carbide (Trade name: Ekasic-F, Comp: ESK Ceramics) has been
brazed to commercial steel (C45E, Matnr. 1.1191) using SnAgTi-filler alloys. The braze pellets were dry pressed based
on commercially available powders and polished to a thickness of 300 μm. The SnAgTi-fractions were varied with the
objective of improving the compound strength. Furthermore, tungsten reinforced SnAgTi-fillers were examined with
regard to the shear strength of the ceramic/steel joints. Polished microsections of SnAgTi-pellets were investigated
before brazing in order to evaluate the particle distribution and to detect potential porosities using optical microscopy.
The brazing temperature and the influence of the reinforcing particles on the active braze filler were determined by
measurements with a differential scanning calorimeter (DSC). After brazing. the ceramic-steel joints were characterized
by scanning electron micrographs and EDX-analysis. Finally the mechanical strength of the braze-joints was determined
by shear tests.
In order to develop a multifunctional material, a laser induced process was applied to change the properties of a glass-ceramic
by introducing a second phase into the surface. Localized melting of the ceramic and/or a melting of a preplaced
powder layer was achieved by the application of laser energy. After solidification a composite with new properties was
developed. The characteristic feature of the process is the option of a local modification, which is restricted to the
substrate surface and can be controlled by adjustment of the laser parameters. Accordingly modified areas with different
geometries and with a complex multiphase microstructure could be fabricated, while the ceramic bulk remains in its
original state.
Sintered LTCC-substrates (Low Temperature Co-fired Ceramic) were modified with powders metal-oxides (WO3, CuO)
with nanosized particles. Powders of metals (Cu, Ni) were used too. Cladding layers located at the top of the substrate or
layers with a thickness up to several hundred microns, which were embedded into the substrate surface, could be
fabricated. The properties of the laser modified regions differ significantly from that of the LTCC-substrate. The
obtained structures offer modified mechanical, thermophysical and electrical properties. In particular an enhanced
thermal conductivity could be detected. The electrical resistivity of the laser modified tracks widely varied depending on
the process parameters and the powder. Tracks made with CuO- and WO3-powders show a negative temperature
coefficient for electrical resistance, i.e. it decreases with increasing temperature, which is typical for semiconductors.
A laser based process was developed for brazing different ceramics to steel with the focus on the improvement of the
wetting behavior of silicon carbide SSiC which exhibits a poor wetting behavior in comparison to oxide ceramics.
Therefore, the wetting behavior of this SSiC was investigated with different active braze fillers in detail. Two
commercially available braze fillers were used, an AgCuTi - foil and an AgCuInTi - foil and SnAgTi - pellets produced
in our institute in varying compositions were examined concerning their wetting and joining behavior towards SSiC. For
improving the wetting behavior surface structures and conditioning were diversified additionally.
The ceramic - steel - compounds were heated up by CO2 - laser - scanning - system. The steel sided heating calms the
thermal shock for the ceramic sample and the local heat input concentrating on the joining area reduces the compound
stresses to a minimum.
The interfacial areas of the wetted ceramic surfaces and of the joined ceramic - steel systems were evaluated by
microscopic investigations of polished cross-sections. For a better understanding of the brazing process measurements
with a differential scanning calorimeter (DSC) have been performed. Moreover the compound strength of the
brazed joints was determined by shear tests. In context of the strength investigations the influence of different laser
induced structures (Nd:YAG) of the ceramic surfaces on the shear strength was evaluated.
Laser supported processes can be used to modify the properties of ceramic substrates locally. These
processes are characterised by a strong thermal interaction between the laser beam and the ceramic
surface which leads to localised melting. During the dynamic melting process second phase particles are
introduced into the melt pool in order to modify the physical properties. LTCC (Low Temperature Co-fired Ceramics)-substrates were laser alloyed and coated by laser cladding using nanoscaled powders of WO3 and CuO. Depending on the process parameters and the powders used modified areas with different geometries could be fabricated with a complex multiphase microstructure. Particle agglomerates, small crystals as well as grains covered with reaction phase could be found inside the microstructure, in parts with typical length scales in the submicron range. The properties of the laser modified tracks differ significantly from that of the substrate. In particular the thermal and electrical properties were changed. An enhanced thermal conductivity could be detected in laser tracks alloyed with the nano-scaled CuO- and WO3-powders. The electrical resistivity showed a semiconducting behaviour with a negative temperature coefficient, i.e. it decreases with increasing temperature.
Laser supported processes can be used to modify the electrical and thermal properties of ceramic substrates locally.
These processes are characterised by a strong thermal interaction between the laser beam and the ceramic surface which
leads to localised melting. During the dynamic melting process metal particles are introduced into the melt pool in order
to modify the physical properties. Different alumina samples were treated with metal powders of tungsten, copper, and
oxides of these metals. The interface between the metal and the ceramic can be designed by using selected combinations
of metal- and metal-oxide-powders and also by a thermal post-processing. The application of nano-particles during the
laser-dispersing process resulted in completely different characteristics of the micro-structure and the electrical
properties compared to the conventional metal powders with an average grain size of 5 - 15 microns. The micron sized
metal particles are embedded within the ceramic matrix as particle agglomerates or as distinct metal phase the nano-particle
phase covers the grain boundaries of the ceramics leading to network of nano-scaled electrically conducting
"wires". The resulting resistance of the laser tracks can be adjusted from semi-conducting to metallic behavior with a
resistivity down to 2x10-6W/m. The modified ceramic can be used for heating elements working at operation
temperatures of up to 1000oC, high current resistances which can be loaded with currents of up to 100 A.
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