Online monitoring of laser remelting of plasma sprayed coatings to study the effect of cooling rate on residual stress and mechanical properties

This investigation deals with laser remelting of plasma sprayed alumina and chromia coatings. The time-temperature history of the laser remelted zone was recorded using an infrared pyrometer during the remelting operation. Cooling rates, under varying scanning speed, were determined from the time temperature curve. Surface morphology, microstructure, and phases of the laser treated and as-sprayed coatings were characterized using scanning electron microscopy, optical microscopy, X-ray diffraction, respectively. X-ray diffraction was also employed to measure the surface residual stress of the coatings. Inherent features of plasma sprayed coatings like porosity and inter-lamellar boundary were obliterated upon laser remelting. A columnar grain growth perpendicular to the laser scanning direction was observed. The range of roughness of the as-sprayed coatings reduced from 6 to 8?µm to 1–2?µm in the remelted layers. For both coatings, more than 90% reduction in porosity was found upon laser remelting. Surface residual stress of the as-sprayed alumina and chromia coatings was found to be tensile and compressive, respectively. Within the limits of the testing condition the tensile residual stress of the remelted layers increased by up to around 500% in the alumina coatings. In the chromia coating a decrease of compressive stress by up to around 80% was recorded. In the remelted layer the tensile nature of the stress showed a tendency to increase with an increase in the cooling rate. However, the state of stress of the as-sprayed layer, i.e., tensile or compressive, was retained in the remelted layer. The residual stress was found to decrease in the remelted layer with an increase in the degree of overlap of the remelted tracks.     

Additively-manufactured multiphase coatings with laser protection ability for the repairing of MoSi2-SiC/SiC coated C/C composites

The Si-SiC-MoSi₂ and Si-SiC coatings were proposed to repair the damaged MoSi₂-SiC/SiC coated C/C composites by laser directed energy deposition. Laser ablation was used to assess the repair effect. Results showed that both the repaired coatings with dense structure could restore the geometric size of damaged area. Compared with the Si-SiC-MoSi₂ coating, the Si-SiC repaired coating with higher laser reflectivity and more free Si could reduce the heat generation and enhance the heat dissipation during ablation, which lowered the maximum temperature by 347.49 K and 810.77 K under 300 W and 500 W ablation for 7 s separately, beneficial to avoid the secondary laser damage of the repaired area.     

Research of laser remelting on the thermal-mechanical behaviors and heat treatment of yttria-stabilized zirconia coatings

In this study, the thermal and mechanical behaviors were investigated by simulating laser remelting of atmospheric plasma-sprayed yttria-stabilized zirconia coatings, and the molten depth and regions of stress concentration were compared between simulation and experiment. The heat  treatment process of the remelted coating was also simulated. The crack formation mechanism in the YSZ coating remelted by laser and the heat-treatment effect on residual stress were investigated. Results showed that the simulated results were consistent with the experimental measurements, and the residual thermal stress was the main cause of cracks formation. The coating remelted by a laser power of 1500 W and a scanning rate of 9 mm/s possessed less residual concentrated stress and segmented cracks. Heat treatment released concentrated stress, which was still accurate for the ceramic coating. If the coatings were slowly heated to demonstrate heat treatment after laser remelting, the cracks in the remelted layer decreased correspondingly.     

Two-step femtosecond laser etching for bulk micromachining of 4H–SiC membrane applied in pressure sensing

A method of two-step laser etching for bulk micromachining of 4H–SiC membranes through a femtosecond (fs) laser with a wavelength of 532 nm, a pulse width of 290 fs and a repetition rate of 100 kHz is present in this paper. Using a control variable method, the first step of fs-laser etching for rapid material removal and the second step of fs-laser etching mainly for improving surface morphology are studied by changing the laser fluence, scanning spacing, number of scans and scanning speed. The average surface roughness of the membrane bottom after the first step of laser etching is 691 nm, which is reduced to 237 nm after the second step. Compared with one-step laser etching, two-step laser etching obtains a better surface morphology, and it only takes 38% of the processing time of one-step laser etching. The finite element analysis shows that the sensitivity of the SiC pressure sensor drops with the decrease of the membrane roughness. The sensor sensitivity of the membrane fabricated by two-step laser etching is only 3.0% lower than that of the ideal smooth model. Furthermore, a 4H–SiC laser-prepared membrane with a diameter of 1.2 mm and a thickness of 75 μm applied in pressure sensor is tested, showing good linearity and repeatability at a load of up to 10 MPa.     

Micro-welding of sapphire and metal by femtosecond laser

A direct joining of sapphire and Fe–36Ni alloy was successfully realized via femtosecond laser micro-welding for the first time. A sound joint without any voids or microcracks was obtained with a narrow interface width less than 1 μm. There was no obvious element diffusion or metallurgical reactions at the interface. Sapphire and Fe–36Ni alloy were found chemically bonded and mechanically interlocked evidenced by jagged feature at the interface due to “cold” machining of femtosecond laser ablation. The highly localized femtosecond laser irradiation and smaller heat-affected zone contributed to the shear strength of the joint as high as 108.35 MPa. A higher laser scanning speed corresponded with less jagged feature and thermal stress due to the reduced thermal deposition at the interface. Proper micro-welding parameters were obtained for sapphire/Fe–36Ni alloy, and was verified in the direct joining of sapphire/steel and sapphire/silicon. It appears the femtosecond laser micro-welding technique is promising for direct joining of materials with large physical property disparities, and beneficial for manufacturing of optomechanical components at high precision, efficiency and performance.     

Research on the ablation mechanism and ablation threshold of CMC-SiCf/SiC by using dual-beam coupling nanosecond laser

Due to the excellent properties of high hardness, oxidation resistance and high temperature resistance, silicon carbide fiber silicon carbide ceramic matrix composite (CMC-SiC_f/SiC) is a typical difficult-to-process material, and is a high-performance advanced material in the aerospace field. In this paper, two groups of ablation experiments (experiment 1 and experiment 2) were performed on CMC-SiC_f/SiC using a dual-beam coupling nanosecond laser, and the ablation morphology was observed by confocal laser microscope. The dual-beam coupling angle of experiment 2 is obtained by experimental method. And through the method of calculation, we get the dual-beam coupling angle of experiment 1 and experiment 2. According to the dual-beam coupling ablation mechanism, based on the theoretical calculation model of non-destructive method D²-lnP₀, combined with the Equivalent Diameter Calculation Method (EDCM) and Equivalent Area Calculation Method (EACM), the laser ablation threshold corresponding to different beam waist size was calculated and compared. The results show that the ablation region of CMC-SiC_f/SiC surface can be divided into three parts: ablation boundary, recast layer area and SiO₂ coverage area. When the pulse energy increases gradually from 300 μJ to 1500 μJ, the variation trend of hole depth is first increase, second decrease, increase again, and finally decrease. The angle between two laser beams affects the waist radius, which in turn affect the laser ablation threshold. The waist of the dual-beam coupling is elliptical, and the orifice of the ablation hole is elliptical. When the waist radius of nanosecond laser is 57 μm, the laser ablation threshold is calculated to be 3.12 J/cm². The main factors affecting the laser ablation threshold are laser pulse repetition frequency (f), beam waist radius (ω₀), laser pulse width (τ), minimum laser power (P_th), and laser wavelength (λ).     

Influence of preheating processes on the microstructure of laser glazed YSZ coatings

Laser glazing is considered to be a promising surface sealing technique for thermal barrier coating. The dense top layer with reduced surface roughness and the segment cracks perpendicular to the surface are considered to be suitable for improving the thermal cycling and hot corrosion resistance of these kind of coatings. In present study, yttria stabilized zirconia ceramic coatings were manufactured by atmospheric plasma spraying and then subjected to a Nd-YAG pulsed laser source. During the laser glazing process, coatings were preheated to 600 °C and 800 °C in order to obtain different microstructure of the laser glazed coatings. The surface morphologies and cross-sections of the coatings were examined by scanning electron microscopy and microhardness measurements of coatings were carried out. The results indicate that preheating process induces a reduction of the grain size of laser glazed coatings in conjunction with an increasing of microhardness and toughness. In addition, preheating also decreases the substrate-coating interface tensile stress which leads to a reduction of crack surface density.     

Femtosecond laser micromachining in combination with ICP etching for 4H–SiC pressure sensor membranes

4H–SiC is one of the most promising materials for pressure sensing in harsh environments. A Yb:KGW femtosecond laser was employed to fabricate 4H–SiC sensor membranes with size of Φ1200 × 80 μm. The optimal parameter combination under 15 μJ single pulse energy was obtained with the laps of 16, the scanning speed of 130 mm/s, the scanning line interval of 2 μm and the repetition rate of 100 kHz. High size accuracy (±1%) and steep sidewall (87.4°) were achieved. Wet cleaning and inductively coupled plasma (ICP) etching can obviously improve the membrane bottom surface morphology. The surface roughness Ra in X direction was reduced from 0.82 μm to 0.15 μm, and that in Y direction was reduced from 1.32 μm to 0.16 μm. Pinhole defect was related to the nonuniform distribution of laser fluence. This defect can be avoided by reducing the laser spot overlap ratio. Energy-Dispersive X-ray Spectroscopy (EDS) and Raman spectrum were adopted to analyse the changes of material properties after laser processing. The analysis indicated that the crystal properties of the membrane bottom and the thin epitaxial layers on the front side of membrane are not damaged by the integrating micromachining. The results indicate the potential of utilizing the femtosecond laser combined with ICP etching to fabricate 4H–SiC sensor membranes.     

Thermal residual stress gradients in an alumina–zirconia composite obtained by electrophoretic deposition

In order to understand the mechanical behavior of layered composites with compositional gradient, it is necessary to determine their state of residual stresses. Compositionally graded materials can offer the advantage of eliminating abrupt changes in composition between layers having different thermal expansion coefficient. The existence of a compositional gradient can reduce discontinuities in thermal residual stresses, something beneficial from the point of view of the mechanical properties.We present here a study of the microstructure and state of residual stressses in a layered material made of homogeneous layers of alumina and alumina–zirconia separated by thin (less than 300 μm) intermediate compositionally graded layers. The composite was obtained by controlled deposition of powders from solution using an electrophoretic deposition (EPD) method. The phase distribution and compositional gradient in the sintered composite were investigated using scanning electron microscopy (SEM). Thermal residual stresses generated during cooling after sintering were measured by using fluorescence ruby luminiscence piezo-spectroscopy and the profile of hydrostatic stress on alumina was determined at steps of about 300 μm along the direction of the compositional gradient, and at steps of about 30 μm in the compositionally graded layers. The obtained profile of hydrostatic stresses on alumina grains follows closely the profile of compositional changes along the layered composite. The presence of thin intermediate graded layers reduce significantly changes in stress in the layered composite.     

Comparative study on effect of oxide thickness on stress distribution of traditional and nanostructured zirconia coating systems

Numerical based assessment of traditional and nanostructured yttria stabilized zirconia (YSZ) thermal barrier coating systems (TBCs) has been carried out with varying thickness of thermally grown oxide (TGO). Radial, axial and shear stresses are determined for both coatings and are presented in comparison with few novel and interesting results. Elastic strain energy for TGO failure assessment is determined from calculated stress within TGO for varying thickness. Radial stresses at TGO/bond coat interface and maximum axial stresses in nanostructured zirconia coatings are found to be lower than in traditional YSZ up to a critical TGO thickness of 6 –7 μm, after which stresses in nanostructured zirconia coatings increase considerably. However, radial compressive stresses in nanostructured TBCs are lower in all TGO thickness cases and shear stresses are slightly higher with relatively more prominent difference at high oxide thickness.     

Direct selective laser sintering of silicon carbide: Realizing the full potential through process parameter optimization

Direct-Selective Laser Sintering (D-SLS) is a promising Additive Manufacturing (AM) technology for Silicon Carbide (SiC). The appropriate values for the process parameters should be employed to achieve considerable success in the D-SLS of SiC. These process parameters include laser power, scanning speed, layer thickness, hatching space, and compaction ratio. A numerical model has been developed to determine the optimal process parameters for the D-SLS of SiC to be used as a guide during the experimental research. SiC samples were successfully printed using the estimated parameters from the numerical model, proving the reliability of the developed numerical model. With varying layer thicknesses of 22, 30, and 40 μm, the scanning speed at various values, including 100, 250, and 500 mm/s, was investigated. It was possible to print SiC samples with a relative density of 82% directly on a metallic base plate using low scanning speeds and layer thicknesses as low as 22 and 30 μm. This result was the first study to print SiC directly on a metallic baseplate. This remarkable finding will enable many applications that previously required metallic and ceramic materials to adhere together during SLS/M-based 3D printing. The process parameters were optimized to achieve a relative density of 87%, resulting in a laser power of 45 W, a scanning speed of 100 mm/s, and a hatching space of 40 μm. The evaluation of mechanical performance should be considered a future study.     

Cr3+ microspectroscopy measurements and modelling of local variations in surface grinding stresses in polycrystalline alumina

Surface residual stresses caused by grinding and polishing of alumina are thought to influence materials properties but have previously been measured only by low spatial resolution techniques which sample average stresses. In this work confocal Cr³⁺ fluorescence microscopy has been used to investigate the spatial distribution of the residual stresses. A model for the residual stresses, accounting for both surface plastic deformation and “pullout” of material from the surface by brittle fracture, was developed to help in analysing the results. After coarse diamond grinding, the results showed that the residual stresses fluctuate greatly with position. Large tensile stresses (up to ～600 MPa) were found below the plastically deformed surface layer in regions between the “pullouts”. These tensile stresses are expected to aid crack propagation and further surface pullout. They arise because pullout removes parts of the plastically deformed surface layer. The stresses beneath the pullout sites themselves were compressive, but the largest compressive stresses (≈?1.5 GPa) were within the plastically deformed surface regions and extended to a depth of 1.3 μm. The plastically deformed surface layer was much shallower following polishing with 3 μm diamond paste but the compressive stress within it was of similar magnitude to that in the plastically deformed surface layer caused by grinding.     

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.     

Surface microstructure evolution of Cf/SiC ceramic matrix composite microgrooves processed by ultrafast laser

Modern aviation components have higher requirements for high temperature resistance, high strength and lightweight materials, and ceramic matrix composites have superior overall performance. However, its high brittleness and anisotropy lead to a challenge for manufacturing. In order to understand the formation conditions and the evolution of surface microstructures of the C_f/SiC microgrooves processed by ultrafast laser comprehensively, we designed a single-factor experiment and performed sensitivity analysis. The experiment results showed that the pulse energy had great effects on the depth of the microgroove, and the intense ablation caused more active oxidation of SiC to occur, generating more SiO(g). However, too much pulse energy may cause the material removal mechanism to be more due to the photothermal effect rather than the plasma effect. Low repetition frequency caused a large number of laminated connections in the microgroove and the oxide gradually changed from lumpy to flocculent as the repetition frequency increased. The more scanning times, the more ablation products sputtered onto the sample surface, including unablated carbon fibers. Shallow depth and ablation residues remained in the microgroove occurred under few scanning times. Although too fast scanning speed leaded to a rapid decrease in the microgroove depth, too slow scanning speed also generated more unablated carbon fibers sputtering out of the microgrooves. The microgroove depth had the highest sensitivity to the repetition frequency, followed by the pulse energy and scanning speed. The pulse energy and scanning speed had a greater effect on the oxide layer height, the repetition frequency affected the oxide layer width, and the scanning speed affected the microgroove width significantly. According to the processing requirements and the hot spot map, the processing parameters that can be adjusted effectively will be able to be obtained.     

Reduction of graphite oxide to graphene with laser irradiation

Reduction of graphite oxide (GO) to graphene induced by picosecond pulsed laser irradiation has been studied by Raman spectroscopy, scanning electron microscopy together with modeling of temperature dynamics in the materials. Dependence of the D, G, and 2D Raman band parameters on the laser pulse energy and the irradiation dose was evaluated. The exponential decline of the full width at half maximum of the Raman lines with increasing product of the pulse energy and irradiation dose was observed indicating ordering in the film and reduction in the number of graphene layers during the laser treatment. The minimum concentration of structural defects and the largest relative intensity of the 2D peak were found for the 50 mW mean laser power and the 30 mm/s scanning speed. Modeling of temperature dynamics revealed that the temperature of the GO film irradiated with a single laser pulse at a fluence of 0.04 J/cm² (50 mW) increased up to 1400 °C for a few nanoseconds, which was sufficient for the effective reduction of GO to graphene with successive laser pulses.     

Thermal and mechanical properties of crack-designed thick lanthanum zirconate coatings

Vertical cracks are beneficial in thermal barrier coatings due to enhanced thermo-mechanical compliance. Accordingly, an aqueous nitrate based precursor solution was atomized on stainless steel substrates by spray pyrolysis to deposit thick crack-designed lanthanum zirconate coatings. Coatings with designed crack patterns were deposited and characterized by electron microscopy, tribology, Vickers indentation, and thermal diffusivity. The crystallization of the coatings was investigated by in situ high temperature X-ray diffraction. The green coatings crystallized from 600 °C and the pyrochlore structure was formed after heat treatment at 1000 °C. Crystalline lanthanum zirconate multilayered coatings with small crack spacing and crack opening exhibited a higher density, a higher hardness, lower thermal diffusivities, and higher thermal conductivities compared to crystalline monolayered coatings of similar thickness with large crack spacing and crack opening. The thermal diffusivity of the coatings, ∼28 mm²/s at room temperature, was similar to the values reported for yttria-stabilized zirconia plasma sprayed coatings.     

Residual stress effect on the fracture toughness of lithium disilicate glass-ceramics

In glass-ceramics (GCs), on cooling from the crystallization temperature, internal residual stresses are generated due to the difference between the thermal expansion coefficient (TEC) of the crystal phase(s) and the residual glass. These stresses could degrade or promote their mechanical properties. In this work, we varied the magnitude of the residual stresses in lithium silicate GCs by designing their microstructures. The level of internal stresses was measured using (Synchrotron) X-ray diffraction. The effects of anisotropy of thermal expansion, crystal shape, and intensity of the residual stresses were analyzed and compared using theoretical models. We extended the Hsueh-Becher model to include the thermal expansion anisotropy of the orthorhombic lithium disilicate (LS2) crystals. We found that the average residual stresses within the LS2 crystals are compressive or null (−100 to ~0) and highly anisotropic. Most importantly, within the limits of this study, we found no evidence for the influence of (compressive or null) residual stresses on the fracture toughness of the studied GCs. Within the crystal size range from 1 to 5 μm, a highly crystallized volume fraction coupled to relatively large crystals (5 μm) of high elastic modulus improved the glass-ceramic fracture toughness. This result can guide the microstructural design of novel tough GCs.     

Laser surface modification of porous yttria stabilized zirconia against CMAS degradation

Here, we present a new combined freeze-casting and laser processing method for the design of yttria-stabilized zirconia (YSZ) based thermal-barrier coatings. YSZ ceramics with unidirectionally-aligned pore channels were created using the freeze-casting method. After sintering, top view and cross-sectional scanning electron microscopy (SEM) revealed the structural features of the preform, which exhibits a 74 ± 2% volume fraction of porosity and an average pore channel size of 30 ± 3 μm. The measured thermal conductivity of this porous structure was 0.27 ± 0.02 W/(m K), which is eight times lower than that of reported values for dense YSZ. Though high porosity is beneficial both from a structural and thermal response perspective, the open porosity could potentially be an issue from an application stand-point when evaluating the resistance of materials to calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS attack, which can originate from deposits of molten sand, ash, and dust, is one of the major causes of thermal barrier coating failure. Therefore, the surface of the porous samples was modified using a laser process to create a barrier to CMAS infiltration. SEM micrographs aided in determining the optimum laser parameters required to fully seal the surface using a laser treatment. The performance of the original porous and surface-modified YSZ was compared by conducting CMAS infiltration studies. Laser modification was shown to be a viable technique to significantly reduce CMAS infiltration in porous thermal barrier coatings.     

Fabrication of grooves on polymer-derived SiAlCN ceramics using femtosecond laser pulses

In this study, grooves were fabricated on the surface of fully dense polymer-derived SiAlCN ceramics. Industrial femtosecond laser source was used at wavelength of 1030 nm with pulse duration of 290 fs and repetition rate of 100 kHz. Moreover, comprehensive study was carried out to evaluate the influence of scan speed and energy fluence on grooves quality, including the heat-affected zone around the laser-machined grooves, microstructures of laser-irradiated surface, and cross-section morphology of grooves. A series of grooves with width in the range of 30–80 μm and depth below 280 μm was successfully fabricated using femtosecond laser pulse. Laser parameters were optimized to obtain grooves with satisfying surface quality. Furthermore, formation and disappearance of laser-induced periodic surface structures were systematically investigated. This study proposes fabrication of grooves on SiAlCN ceramics via laser processing, which provides precise method for fabrication of microstructure with fascinating properties.     

Femtosecond Ti: Sa ultra short-pulse laser irradiation effects on the properties and morphology of the zirconia surface after ageing

Femtosecond pulses from a Ti:Sapphire laser were used to irradiate specimens of yttria-stabilised (35% mol) tetragonal zirconia (Y-TZP) with the purpose of studying the effects of the irradiations on their surface properties and morphology after ageing. Zirconia disks were divided into eight groups (n = 32) according to their surface treatment and subsequent ageing: Control: no treatment; sandblasting: Al₂O₃ sandblasting 50 μm; and ultrashort laser pulses irradiation with 25 μJ pulses, considering two different scanning steps based on the width between two grooves. These groups were duplicated and submitted to ageing. The surfaces were analysed using scanning electron microscopy (SEM), and X-ray diffraction. A finite element analysis, a biaxial flexure test, as well as fractographic and Weibull analyses, were performed. The strengths of the disks were statistically different for the treatment factor, and the principal stresses seemed to be concentrated at the centre of the specimens, as predicted by the computer simulations. Ageing decreased the strengths for all groups and increased the Weibull modulus for the laser group with the 40 μm-width between two grooves. The sandblasting group presented the highest monoclinic phase peak. Although the most significant strength was found within the sandblasting group, the phase transformation was favourable to the laser groups. The Weibull modulus was higher for the laser group with the 60 μm-width between two grooves, confirming the highest homogeneity of its failure distribution. Regardless of the surface treatment, strength was decreased with ageing in all groups. The femtosecond Ti:Sa ultra-short pulse laser irradiation can be suggested as an alternative to the gold standard sandblasting in long-term Y-TZP zirconia rehabilitations, such as crowns and veneers.