Wear assessment of Cr2O3-/TiAlN-coated DAC-10 tool steel against steel and Al2O3 counterbodies

TiAlN film was deposited on Cr₂O₃-coated plasma-nitrided DAC-10 tool steel to obtained multilayer Cr₂O₃/TiAlN coating layer using cathodic arc deposition technique. The structural make-up of the coating was characterized using Atomic Force Microscopy (AFM) and X-ray diffraction methods, and the mechanical properties were evaluated using nanoindentation and nanoscratch test. The structural phases of the coating indicated the presence of crystalline CrO structure and cubic TiAlN phases. The coating showcased improved hardness (38 GPa), elastic modulus (387 GPa), and adhesion along with appreciable H/E (0.09) and H³/E² (0.366 GPa) attributes. Further, friction-induced wear behavior of the coating was investigated against steel and Al₂O₃ counterbodies under dry sliding conditions. The wear behavior of the coating was greatly influenced by its hardness and deformation properties and frictional behavior of the counterbodies. More spikes and fluctuation were observed in the frictional curve against Al₂O₃ counterbody attributed to the emanation of TiO₂, Cr₂O₃, and Al₂O₃ compounds due to dry sliding leading to the formation of flakes and delamination induced debris. Against the steel counterbody, the coating mainly formed a typical smooth glossy surface ascribed to the formation of Fe₂O₃ compound on the worn surface.     

Dry sliding wear behaviour of ZrB2-based ceramics: Self-mated and cross coupling with alumina

Systematic dry sliding wear tests with monolithic ZrB₂ and Al₂O₃ pins coupled to ZrB₂, ZrB₂-20 vol% SiC and Al₂O₃ discs were carried out in a disc-on-pin configuration. The steady state friction of ZrB₂ self-mated or cross coupled with Al₂O₃ was about 1.1. Self-mated monolithic ZrB₂ discs worn about three orders of magnitude more than self-mated Al₂O₃ discs. ZrB₂ pin wear rate was almost double when coupled to ZrB₂ or ZrB₂-20 vol% SiC discs than when coupled to Al₂O₃ discs. The wear track of ZrB₂-based materials showed an oxygen increment due to humidity-driven tribo-reaction. In all the systems, the main wear mechanisms observed were microfracture and abrasion. Numerical calculations and fracture models were employed to describe the wear mechanisms. By nanoindentation tests on worn and unworn areas, a significant lower hardness of the debris layer formed when ZrB₂ materials were involved.     

Research on the tribological performance of Cr2O3 filled with bronze‐based PTFE composites

Enhancement of the wear resistance of bronze‐filled polytetrafluoroethylene (PTFE) composites has been achieved using various fillers, for example, chromic oxide (Cr₂O₃), molybdenum disulfide (MoS₂), graphite, and nanometer aluminum oxide (n‐Al₂O₃), in the present study. The wear resistance was evaluated by a block‐on‐ring wear tester, and the effects of fillers on the wear resistance as well as the mechanism were investigated. The wear rate for the composite where the recipe containing 59% PTFE + 40% bronze + 1% Cr₂O₃ was 0.5 × 10^?5 mm^?3/N m and for the composite in the recipe containing 60% PTFE + 40% bronze was 4.2 × 10^?5 mm^?3/N m, which meant that that Cr₂O₃ increased the wear resistance by approximately 10 times. The differential scanning calorimetry measure analysis showed that Cr₂O₃ had a positive effect on the crystallization of PTFE; the crystallinity of PTFE composites increased from 45% to 52%, which exhibited improved wear resistance. Wear testing and scanning electron microscope analysis had shown that Cr₂O₃ had a positive effect on the formation of transfer film and keeping it stable to exhibit improved wear resistance. X‐ray photoelectron spectroscopy results also showed that Cr₂O₃ was effective in tribochemical reactions during sliding against stainless ring; these maybe responsible for forming transfer film and lowering wear rate of composite. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41117.     

Spark plasma sintering of Al-doped ZrB2–SiC composite

This research presents the influence of Al addition on microstructure and mechanical behavior of ZrB₂–SiC ultra-high temperature ceramic matrix composite (UHTCMC) fabricated by spark plasma sintering (SPS). A 2.5?wt% Al-doped ZrB₂–20?vol% SiC UHTCMC was produced by SPS method at 1900?°C under a pressure of 40?MPa for 7?min. The microstructural and phase analysis of the composite showed that aluminum-containing compounds were formed in-situ during the SPS as a result of chemical reactions between Al and surface oxide films of the raw materials (i.e. ZrO₂ and SiO₂ on the surfaces of ZrB₂ and SiC particles, respectively). The Al dopant was completely consumed and converted to the intermetallic Al₃Zr and Al₄Si compounds as well as Al₂O₃ and Al₂SiO₅. A relative density of 99.8%, a hardness (HV5) of 21.5?GPa and a fracture toughness (indentation method) of 6.3?MPa?m^1/2 were estimated for the Al-doped ZrB₂–SiC composite. Crack bridging, branching, and deflection were identified as the main toughening mechanisms.     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

Phase Formation in the ZrO2 – Nd2O3 (Y2O3) – Al2O3 (Cr2O3) SYSTEMS AND PROPERTIES OF MATERIALS BASED ON THEM

The nature and degree of completeness of phase transformations that occur at 1100 – 1750°C in ZrO₂ – Nd₂O₃ (Y₂O₃) – Al₂O₃ (Cr₂O₃) compositions rich with ZrO₂ are studied. It is shown that the zirconium-yttrium cubic solid solution has a maximum resistance to the action of Al₂O₃ and Cr₂O₃. The sinterability and other properties of parts from ZrO₂ can be controlled within a quite wide range by adding Al₂O₃, Cr₂O₃, and NdCrO₃ to ZrO₂ stabilized by Nd₂O₃.     

Catalytic Ozonation of m-Dinitrobenzene

The efficiency of catalytic ozonation with homogeneous (containing dissolved ions of Fe²⁺, Mn²⁺, Cu²⁺, Ni²⁺, Co²⁺, V⁵⁺, Cr³⁺, Mo⁶⁺) and heterogeneous (MnO₂, Ni₂O_3, Fe₂O₃, CuO, Al₂O₃, CoO, V₂O₅, Cr₂O₃, MoO₃, TiO₂) catalysts and non-accompanied ozonation was compared for degradation of m-dinitrobenzene (m-DNB). Several transition metals in homogeneous and heterogeneous form improved significantly the ozone performance for degradation of m-DNB. This improvement was found to be due to supplementary formation of reactive species (hydroxyl radicals) and better ozone utilization. The effects observed were found to be strongly dependent on the treatment conditions.     

Point contact abrasive wear behavior of MAX phase materials

The room temperature abrasive wear behavior of three selected MAX phases, Ti₃SiC₂, solution strengthened Ti_2.7Zr_0.3SiC₂ and Cr₂AlC, is investigated by low velocity scratch testing using a diamond conical indentor with a final radius of 100 μm and a cone angle of 120° and applied loads of up to 20 N. All three materials showed a relatively low wear resistance in comparison to most engineering ceramics such as Al₂O_3, Si₃N₄ and SiC. For all three materials, the wear rate scaled more or less linearly with the applied load. The softer Ti₃SiC₂ with a hardness of 2.8 GPa showed the lowest wear resistance with extensive ploughing and grain breakout damage, both within and outside the direct wear track, in particular at the highest load. The hardest material, Ti_2.7Zr_0.3SiC₂, with a hardness of 7.3 GPa, showed a 5 times better wear resistance. The Cr₂AlC with a hardness of 4.8 GPa showed a wear resistance equal to or even better than that of the Ti_2.7Zr_0.3SiC₂. The wear mechanism depends on the applied load and the microstructure of the MAX phase materials tested. For the Ti₃SiC₂ sample, a quasi-plastic deformation behavior occurs below a point load of 10 N, resulting in grain bending, kink band formation and delamination, grain de-cohesion, as well as trans-and intra-granular fracture near the scratch groove. At this load, the Ti_2.7Zr_0.3SiC₂ and Cr₂AlC MAX samples display plastic ploughing, grain boundary cracks and material dislodgments.     

Effect of the calcination temperature on the cation distribution and optical properties of Mg0.5Co0.5CrAlO4 spinel pigment

In this study, a blue-green pigment has been prepared by partially replace Co²⁺ and Cr³⁺ in CoCr₂O₄ spinel structure with Mg²⁺ and Al³⁺ using a gel casting method The gel precursor was calcined at various temperatures (900–1400 °C) to obtain Mg_0.5Co_0.5CrAlO₄ spinel pigment. Combining the Rietveld refinement method of XRD and peak fitting of XPS high-resolution spectra, the relationships between the cation distributions (Co²⁺, Mg²⁺, Al³⁺, and Cr³⁺) in the tetrahedron and octahedron of the spinel structure and the calcination temperature were examined. In the octahedron, the contents of Co²⁺ and Al³⁺ decreased with increasing calcination temperature, and the Mg²⁺ and Cr³⁺ contents exhibited the opposite trend. The bond lengths of A-O and B–O change with increasing calcination temperature, thereby leading to a change in the unit cell. The optical performance of the pigments was investigated via UV–vis and CIE L*a*b* spectrophotometry, and the study shows that the blue-green hue of the pigment powder is caused by the absorption at υ3～υ8 (370 nm–640 nm) in the visible light region. The varied contents of Co²⁺ and Cr³⁺ in the spinel structure among calcination temperatures cause the absorption spectrum intensity change, thereby resulting in various blue-green tones. This study lays the foundations for subsequent investigations of colour modification in Mg_xCo_1-xAl_yCr_2-yO₄ spinel.     

Processing of ZrB2 tribo-ceramics by reactive spark plasma sintering of ZrH2+2B subjected to high-energy pre-ball-milling

The reactive spark plasma sintering (RSPS) of monolithic ZrB₂ ceramics from ZrH₂+2B powder mixtures subjected to shaker pre-milling was investigated, and compared with other three sintering approaches. It was found that RSPS is optimal at 1850 °C, which results in fully-dense ZrB₂ ceramics with ∼20 GPa hardness. Comparatively, at 1850 °C RSPS from the simply-mixed ZrH₂+2B powder mixture, SPS from the commercial ZrB₂ powder, and SPS from the shaker-milled ZrB₂ powder result in non-dense (76.7–86.7%) and softer (6.0–11.8 GPa) ZrB₂ ceramics. Furthermore, the optimally RSPS-ed ZrB₂ ceramic was subjected to unlubricated sliding-wear tests against diamond under 40 N load for 1000 m of sliding, demonstrating that it is a promising tribo-ceramic that only undergoes mild tribo-oxidative wear at 10^–8 mm³/(N·m) in the form of a slight plasticity-dominated two-body abrasion with eventual formation and partial loss of a self-lubricating and protective oxide tribolayer.     

Preparation and Application of Strong Near‐Infrared Emission Phosphor Sr3SiO5:Ce3+,Al3+,Nd3+

This work investigated the near‐infrared (NIR) emission properties of mCe³⁺, xNd³⁺ codoped Sr_3?m?x(Si_1?m?xAl_m+x)O₅ phosphors. Samples with various doping concentrations were synthesized by the high‐temperature solid‐state reaction. Al³⁺ ions have the ability to promote Ce³⁺ ions to enter into the Sr²⁺ sites and to improve the visible emission of Ce³⁺. Thus the NIR emission of Nd³⁺ is enhanced by the energy‐transfer process, which occurred from Ce³⁺ to Nd³⁺. The device based on these NIR emission phosphors is fabricated and combined with a commercial c‐Si solar cell for performance testing. Short‐circuit current density of the solar cell is increased by 7.7%. Results of this work suggest that the Sr_2.95Si_0.95Al_0.05O₅:0.025Ce³⁺, 0.025Nd³⁺ phosphors can be used as spectral convertors to improve the efficiency of c‐Si solar cell.     

Efficient Near‐Infrared Emission of Ce3+–Nd3+ CoDoped (Sr0.6Ca0.4)3(Al0.6Si0.4)O4.4F0.6 Phosphors for c‐Si Solar Cell

Ce³⁺, Nd³⁺ codoped (Sr_0.6Ca_0.4)₃(Al_0.6Si_0.4)O_4.4F_0.6 phosphors were synthesized through the high‐temperature solid‐state reaction method. Luminescence spectra, absorption spectra, and decay lifetimes of these samples have been measured to prove the energy‐transfer process from Ce³⁺ to Nd³⁺. Under UV and blue light excitation, (Sr_0.6Ca_0.4)₃(Al_0.6Si_0.4)O_4.4F_0.6:Ce³⁺,Nd³⁺ phosphors exhibit near‐infrared (NIR) emission, mainly peaking at 1093 nm and secondarily at 916 nm. The NIR emission matches well with the band gap of c‐Si. Results of this work suggest that the (Sr_0.6Ca_0.4)₃(Al_0.6Si_0.4)O_4.4F_0.6:Ce³⁺, Nd³⁺ phosphors have potential application as down‐shifting luminescent convertor for enhancing the photoelectric conversion efficiency of c‐Si solar cell.     

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

It has previously been suggested that Al₂O₃/SiC nanocomposites develop higher surface residual stresses than Al₂O₃ on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr³⁺ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ～ ?2 GPa within the plastically deformed surface layers to ～ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ～ ?2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ～ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the nanocomposites.     

Broadband emission of single-phase Ca3Sc2Si3O12:Cr3+/Ln3+ (Ln = Nd,Yb, Ce) phosphors for novel solid-state light sources with visible to near-infrared light output

Novel single-component phosphors Ca₃Sc₂Si₃O₁₂:Cr³⁺/Ln³⁺ (CSS:Cr³⁺/Ln³⁺, Ln = Nd, Yb, Ce) with broadband near-infrared (NIR) emissions are synthesized. Their phase structure, photoluminescence properties and energy transfer between Cr³⁺ and Ln³⁺ ions are investigated. In the CSS host, Cr³⁺ ions occupy Sc³⁺ sites with low-field octahedral coordination, and thus show a broadband emission in 700–900 nm under the blue light excitation. Nd³⁺, Yb³⁺ and Ce³⁺ ions substitute Ca²⁺ sites in CSS, where Nd³⁺ and Yb³⁺ ions emit the NIR light in 900–1100 nm and their excitation efficiencies at ∼450 nm are greatly enhanced by utilizing the energy transfer from Cr³⁺ to Nd³⁺/Yb³⁺ ions. Ce³⁺ ions can further enhance the absorption of CSS:Cr³⁺/Ln³⁺ phosphors to the blue light, and at the same time contribute to the visible emission in 480–650 nm. Furthermore, CSS:Cr³⁺/Ln³⁺ phosphors show good thermal stability, and approximately 79% of the initial emission intensity is sustained at 150 °C. A phosphor-converted LED (pc-LED) prototype is fabricated by integrating the as-prepared phosphor CSS:Cr³⁺/Ln³⁺ and the commercial phosphor CaAlSiN₃:Eu²⁺ with the blue LED chip, showing a super broadband emission ranging from 450 to 1100 nm. This finding shows the potential application of CSS:Cr³⁺/Ln³⁺ phosphors in broadband NIR pc-LEDs or super broadband LED sources with visible to NIR light output.     

Microstructure and mechanical properties of Al2O3–Ti(C,N)–cBN composites prepared by a novel hot-forging process

Al₂O₃–cBN has received considerable attention in the field of ceramic cutting tools due to its high hardness, high wear resistance, and low cost, but poor interfacial bonding affects the performance of the composite. In this study, a novel hot-forging process was used to prepare high-performance Al₂O₃–cBN composites using Ti(C,N) as a binder. The evolution of the morphology, phase, and microstructure of the hot-forged Al₂O₃–Ti(C,N)–cBN composites was determined, and the mechanical properties were measured. The relative density of the composites increases significantly after hot forging, and the deformation of the composites increases with the hot-forging temperature. The highest performing Al₂O₃–Ti(C,N)–cBN composite was prepared by hot forging at 1600°C and has a hardness of 20 GPa, a bending strength of 647 MPa and a fracture toughness of 5.37 MPa m^1/2, which are superior to those of a directly hot-pressed sintered composite. However, at hot-forging temperatures higher than 1700°C, Al₅O₆N and TiB₂ are formed in the composite. In the composite hot forged at 1800°C, serrated grain boundaries promote the strength and toughness of the composite to 877 MPa and 6.76 MPa m^1/2, respectively. Therefore, the novel hot-forging process is expected to enhance material properties.     

Synthesis,microstructure and mechanical properties of Cr2AlC/Al2O3 in situ composites by reactive hot pressing

Al₂O₃ particle-reinforced Cr₂AlC in situ composites were successfully fabricated from powder mixtures of Cr₃C₂, Cr, Al, and Cr₂O₃ by a reactive hot-pressing method at 1400 °C. A possible synthesis mechanism was proposed to explain the formation of the composites in which Al₂O₃ was formed by the aluminothermic reaction between Al and Cr₂O₃, meanwhile, Cr₃C₂, Al, together with Cr reacted to form Cr₂AlC in a shortened reaction route. The effect of Al₂O₃ addition on the microstructure and mechanical properties of Cr₂AlC/Al₂O₃ composites was investigated. The results indicated that the as-sintered products consisted of Cr₂AlC matrix and Al₂O₃ reinforcement, and the in situ formed fine Al₂O₃ particles dispersed at the matrix grain boundaries. The flexural strength and Vickers hardness of the composites increased gradually with increasing Al₂O₃ content. But the fracture toughness peaked at 6.0 MPa m^1/2 when the Al₂O₃ content reached 11 vol.%. The strengthening and toughening mechanism was also discussed.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

Transition metal diboride-silicon carbide-boron carbide ceramics with super-high hardness and strength

Three phase boride and carbide ceramics were found to have remarkably high hardness values. Six different compositions were produced by hot pressing ternary mixtures of Group IVB transition metal diborides, SiC, and B₄C. Vickers’ hardness at 9.8 N was ~31 GPa for a ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C, increasing to ~33 GPa for a ceramic containing equal volume fractions of the three constituents. Hardness values for the ceramics containing ZrB₂ and HfB₂ were ~30% and 20% lower than the corresponding TiB₂ containing ceramics, respectively. Hardness values also increased as indentation load decreased due to the indentation size effect. At an indentation load of 0.49 N, the hardness of the previously reported ceramic containing equal volume fractions of TiB₂, SiC and B₄C was ~54 GPa, the highest of the ceramics in the present study and higher than the hardness values reported for so-called “superhard” ceramics at comparable indentation loads. The previously reported ceramic containing 70 vol% TiB₂, 15 vol% SiC, and 15 vol% B₄C also displayed the highest flexural strength of ~1.3 GPa and fracture toughness of 5.7 MPa·m^1/2, decreasing to ~0.9 GPa and 4.5 MPa·m^1/2 for a ceramic containing equal volume fractions of the constituents.     

Microstructure evolution during reactive sintering of Y3Al5O12:Nd3+ transparent ceramics: Influence of green body annealing

The effect of green bodies’ mesostructure on the porosity, optical properties and laser performance of reactive sintered Y₃Al₅O₁₂:Nd³⁺ transparent ceramics was studied. Only minor changes in microstructure were revealed for green bodies without annealing and those annealed at 600, 800, 1000 °C, while average pore size increases to 140 nm for sample annealed at 1200 °C. Y₃Al₅O₁₂:Nd³⁺ ceramics sintered at 1750 °C for 10 hours possess significant differences in the final porosity, optical and laser characteristics. Despite all green bodies exhibit a similar phase evolution and sintering behavior on heating, the differences appear in the final stage, when the latest percentage of porosity is removed. The green bodies annealed at 600 °C have an optimal mesostructure from the standpoint of uniform densification. Y₃Al₅O₁₂:Nd³⁺ ceramics prepared using these green bodies exhibit porosity ≤0.001 vol% and yield efficient laser emission at 1.06 μm with slope efficiency as high as 67% in quasi-continuous pumping at 807 nm.     

Effect of ZrB2 content on the densification,microstructure, and mechanical properties of ZrC-SiC ceramics

ZrC ceramics containing 30 vol% SiC-ZrB₂ were produced by high-energy ball milling and reactive hot pressing. The effects of ZrB₂ content on the densification, microstructure, and mechanical properties of ceramics were investigated. Fully dense ceramics were achieved as ZrB₂ content increased to 10 and 15 vol%. The addition of ZrB₂ suppressed grain growth and promoted dispersion of the SiC particles, resulting in fine and homogeneous microstructures. Vickers hardness increased from 23.0 ± 0.5 GPa to 23.9 ± 0.5 GPa and Young’s modulus increased from 430 ± 3 GPa to 455 ± 3 GPa as ZrB₂ content increased from 0 to 15 vol%. The increases were attributed to a combination of the higher relative density of ceramics with higher ZrB₂ content and the higher Young’s modulus and hardness of ZrB₂ compared to ZrC. Indentation fracture toughness increased from 2.6 ± 0.2 MPa⋅m^1/2 to 3.3 ± 0.1 MPa⋅m^1/2 as ZrB₂ content increased from 0 to 15 vol% due to the increase in crack deflection by the uniformly dispersed SiC particles. Compared to binary ZrC-SiC ceramics, ternary ZrC-SiC-ZrB₂ ceramics with finer microstructure and higher relative densities were achieved by the addition of ZrB₂ particles.