Dissimilar Ceramics Research Articles

Direct welding of dissimilar ceramics YSZ/Sapphire via nanosecond laser pulses

The joining of YSZ and sapphire was achieved through the utilization of nanosecond laser pulses. The microstructure, mechanical properties and forming process of the YSZ/Sapphire joint were systematically studied. The joint consists of an amorphous region guided by the temperature gradient and a dendritic recrystallization region. The amorphous region is situated centrally within the joint, while the recrystallization region surrounds it in a distributed manner. The reason for this phenomenon lies in the fact that the welding process occurring in the central region of the joint, and the extreme cooling prevents Al3+, Zr4+ and O2- ions from forming regular crystal phases. During this study when the laser power is set at 9 W and the scanning speed is adjusted to 80 mm/s, optimal welding interface performance can be achieved, resulting in a maximum joint shear strength of 32 MPa. This study provides a new idea for rapid and efficient welding of advanced ceramic.

Superlubricity and running-in wear maps of water-lubricated dissimilar ceramics

Water-lubricated ceramic sliding tribosystems, when operating under superlubricity conditions, reduce frictional energy losses in many applications. Aqueous superlubricity of 5 dissimilar ceramic tribopairs (Si3N4–Al2O3, Si3N4-ZTA, Si3N4–SiC, Al2O3–SiC, ZrO2–SiC) was thus studied in a series of ball-on-disk tests, with the identification of wear mechanisms and lubrication regimes, while introducing a running-in wear map approach to elucidate transient tribological phenomena. All pairs presented superlubricity for certain combinations of normal load and sliding speed, after a running-in period where mechanical and thermal severities of contact remained mostly below their respective threshold values, promoting mild wear regimes. Load-carrying capacity of ZrO2–SiC was the highest due to high homogeneity and low roughness of the tribofilm on SiC disk’s wear track.

Advanced high-temperature (RT-1100°C) resistant adhesion technique for joining dissimilar ZrO2 ceramic and TC4 superalloys based on an inorganic/organic hybrid adhesive

To meet the high demand for ceramic/superalloy composite structural components in various fields, an advanced high-temperature adhesion technique was firstly developed by preparing a novel inorganic/organic hybrid adhesive suitable for ZrO 2 and TC4. Chemical bonding started to work at ∼600°C, and became the crucial bonding mechanism at elevated temperatures. The formation of ZrSiO 4 and Ti 5 Si 3 at the interfaces of two substrates not only increased the interfacial connection strength, but also formed two gradient layers with a size of ∼2 μm to effectively alleviate the difference of composition and performance between the adhesive and substrates. In the temperature range of 500–900°C, the matching degree of CTE among ZrO 2 , adhesive and TC4 is higher, and the maximum difference does not exceed 3×10 -6 K -1 . Meanwhile, the formation of a composite structure containing various ceramics (ZrO 2 , SiC and ZrB 2 ) and intermetallics (Ni–Si, Al–Ni), and the improvement of structural compactness of adhesive from 500 to 900°C greatly improved the bonding strength to the maximum value of 31.4 MPa at 900°C. Also, the adhesive pretreated at 900°C showed good thermal cycling resistance, and the strength was still higher than 15 MPa after 50 cycles. For cured adhesive, when used directly in an extreme environment, it can provide bonding strength not less than 5 MPa in the whole temperature range, indicating that the adhesive possessed potential emergency repair convenience. This work significantly broadened the application of high-temperature-resistant adhesion technology in the connection of dissimilar ceramics and alloys.

Tribological transitions during sliding of zirconia against alumina and ZTA in water

Water-lubricated ceramic bearings are currently being used in engineering applications like water pumps, air compressors and marine systems, due to their higher efficiency, reduced environmental impact and simpler constructive designs. In search of low-cost and high-performance tribosystems, bearings made of dissimilar ceramics are not frequently studied, but have potential to show good tribological compatibility. In this study, tribological behavior of two dissimilar ceramic pairs, ZrO2-Al2O3 and ZrO2-ZTA (10vol% ZrO2), was investigated. Water-lubricated ball-on-disc tests under loads ranging from 6.0 to 24.6 N and sliding speeds from 0.1 to 2.0m/s were performed. Worn surfaces were characterized by optical microscopy, SEM and contact-type profilometry. Thermal (Sc,t) and mechanical (Sc,m) severity of contact models explained the mild to severe wear transition as function of tribosystem's variables such as load, speed, distance, materials’ properties, contact geometry and environment. During mild wear regime, wear rates were smaller than 10⁻⁷ mm³/Nm, friction coefficients remained low (0.1<µ<0.2) and surface roughnesses were reduced during sliding. A mechanical, plasticity-dominated, wear mechanism was observed at ZrO2 balls, while tribochemical wear, mixed with minor ploughing marks and microcracking, prevailed at the wear track of Al2O3 and ZTA discs. For severe wear regime, wear rates of balls and discs were greater than 10−3 and 10−5mm3/Nm, respectively, friction coefficients were high (µ≈0.5) and surface roughnesses significantly increased. Dominant wear mechanisms consisted of plastic deformation, microcracking and material transfer between bodies. The role of water in removing heat from the sliding interface was demonstrated through calculations of contact temperatures and heat partition. The study also showed that it is possible to operate ZrO2 bearings at sliding speeds much higher than previously reported through coupling it with a tribologically compatible counter-body, such as Al2O3 or ZTA, that preferably conducts the heat away from the interface.

Optimization of Crack-Free Polytypoidally Joined Dissimilar Ceramics ofFunctionally Graded Material (FGM) Using 3-Dimensional Modeling

Crack-free joining of Si N and Al O using 15 layers has been achieved by a unique approach introducing Sialon polytypoids as a functionally graded materials (FGMs) bonding layer. In the past, hot press sintering of multilayered FGMs with 20 layers of thickness 500 µm each has been fabricated successfully. In this study, the number of layers for FGM was reduced to 15 layers from 20 layers for optimization. For fabrication, model was hot pressed at 38 MPa while heating up to 1700 C, and it was cooled at 2 C/min to minimize residual stress during sintering. Initially, FGM with 15 layers had cracks near 90 wt.% 12H / 10 wt.% Al O and 90 wt.% 12H/10 wt.% Si N layers. To solve this problem, FEM (finite element method) program based on the maximum tensile stress theory was applied to design optimized FGM layers of crack free joint. The sample is 3-dimensional cylindrical shape where this has been transformed to 2-dimensional axisymmetric mode. Based on the simulation, crack-free FGM sample was obtained by designing axial, hoop and radial stresses less than tensile strength values across all the layers of FGM. Therefore, we were able to predict and prevent the damage by calculating its thermal stress using its elastic modulus and coefficient of thermal expansion. Such analyses are especially useful for FGM samples where the residual stresses are very difficult to measure experimentally.

Joining of highly aluminum-doped lanthanum strontium manganese oxide with tetragonal zirconia by plastic deformation

Aluminum-doped lanthanum strontium manganese oxide, La 0.77Sr 0.20Al 0.9Mn 0.1O 3 (LSAM), was joined to stabilized tetragonal zirconia polymorph YTZP (ZrO 2) 0.97(Y 2O 3) 0.03 by a uniaxial stress (3–6 MPa) and high-temperature (1250–1350 °C) bonding method that initiates grain-boundary sliding between the ceramic components. The flow stress of LSAM was larger than that of La 0. 8Sr 0.2Mn 0. 3 under the same test conditions. Electron microscopy confirmed that intergranular penetration occurred at the joining plane, leading to effective bonding between the two dissimilar ceramics. Raman spectral maps of the joining planes obtained with 2-D Raman microscopy demonstrated the absence of any new phases at the interface.

Strain incompatibility between dissimilar piezoceramics with a penny-shaped interfacial electrode

Debonding between metallic electrodes and piezoelectric materials is a common cracking failure phenomenon observed in piezoelectric devices. In an attempt to explore the failure mechanism, an electroelastic model made of two jointed dissimilar piezoelectric ceramics with a thin circular interface electrode was proposed. Explicit solution to the entire electroelastic field perturbed by the electrode is obtained by solving the associated boundary-value problem. It shows that the electroelastic field near the electrode edge bears an inverse square-root singularity. However, the normal stress, strain, electric displacement, and electric field vanish along the interface out of the electrode, while the corresponding radial components disappear within the electrode surface. Moreover, in the case of two dissimilar ceramics polarized in opposite directions, the normal strain shows strong incompatibility across the electrode surface, and the normal stress reaches its peak value at a distance less than half of the electrode radius above and below the electrode surface. Numerical results are presented for PZT-4 +/PZT-4 − and PZT-4 +/PZT-5H − with a circular interface electrode.

Novel joining of dissimilar ceramics in the Si 3N 4–Al 2O 3 system using polytypoid functional gradients

A unique approach to crack-free joining of heterogeneous ceramics is demonstrated by the use of sialon polytypoids as Functionally graded materials (FGM) as defined by the phase diagram in the system, Si 3N 4–Al 2O 3. Polytypoids in the Al 2O 3–Si 3N 4 system offer a path to compatibility for heterogeneous ceramics. This paper describes successful hot press sintering of multilayered FGMs with 20 layers of thickness 500 μm each. Transmission electron microscopy was used to identify the polytypoids at the interfaces of different areas of the joint. It has been found that the 15R polytypoid was formed in the Al 2O 3-contained layers and the 12H polytypoid was formed in the Si 3N 4-contained layers.

Diffusion bonding of NiZn ferrite and nonmagnetic materials

Diffusion bonding of dissimilar ceramics can be accomplished by the application of pressure only while the ceramics are at a temperature where their respective thermal expansion curves cross. A process for manufacturing magnetic head cores is disclosed wnere a NiZn ferrite is bonded by diffusion co a nonmagnetic ceramic by heating the core pieces to the crossover temperature and them applying the bonding pressure. The pressure can be removed either before or after the bonded pieces are cooled.