91 research outputs found

    Fracture toughness of a zirconia engineering ceramic and the effects thereon of surface processing with fibre laser radiation

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    Vickers hardness indentation tests were employed to investigate the near-surface changes in the hardness of a fibre laser-treated and an as-received ZrO2 engineering ceramic. Indents were created using 5, 20, and 30 kg loads to obtain the hardness. Optical microscopy, white-light interferometry, and a coordinate measuring machine were then used to observe the crack lengths and crack geometry. Palmqvist and half-penny median crack profiles were found, which dictated the selection of the group of equations used herein. Computational and analytical approaches were then adapted to determine the K1c of ZrO2. It was found that the best applicable equation was: K1c = 0.016 (E/H)1/2 (P/c3/2), which was confirmed to be 42 per cent accurate in producing K1c values within the range of 8 to 12 MPa m1/2 for ZrO2. Fibre laser surface treatment reduced the surface hardness and produced smaller crack lengths in comparison with the as-received surface. The surface crack lengths, hardness, and indentation loads were found to be important, particularly the crack length, which significantly influenced the end K1c value when K1c = 0.016 (E/H)1/2 (P/c3/2) was used. This is because, the longer the crack lengths, the lower the ceramic's resistance to indentation. This, in turn, increased the end K1c value. Also, the hardness influences the K1c, and a softer surface was produced by the fibre laser treatment; this resulted in higher resistance to crack propagation and enhanced the ceramic's K1c. Increasing the indentation load also varied the end K1c value, as higher indentation loads resulted in a bigger diamond footprint, and the ceramic exhibited longer crack lengths

    Viability and characterization of the laser surface treatment of engineering ceramics

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    Laser surface treatment of engineering ceramics offers various advantages in comparison with conventional processing techniques and much research has been conducted to develop applications. Even so, there still remains a considerable gap in knowledge that needs to be filled to establish the process. By employing a fibre laser for the first time to process silicon nitride (Si3N4) and zirconia (ZrO2) engineering ceramics, a comparison with the CO2 and a Nd:YAG lasers was conducted to provide fundamental understanding of various aspects of the laser beam-material interaction. Changes in the morphology, microstructure, surface finish, fracture toughness parameter (K1c) were investigated, followed by thermal finite element modelling (FEM) of the laser surface treatment and the phase transformation of the two ceramics, as well as the effects of the fibre laser beam parameter - brightness (radiance). Fibre and CO2 laser surface treatment of both Si3N4 and ZrO2 engineering ceramics was performed by using various processing gases. Changes in the surface roughness, material removal, surface morphology and microstructure were observed. But the effect was particularly more remarkable when applying the reactive gases with both lasers and less significant when using the inert gases. Microcracking was also observed when the reactive gases were applied. This was due to an exothermic reaction produced during the laser-ceramic interaction which would have resulted to an increased surface temperature leading to thermal shocks. Moreover, the composition of the ceramics was modified with both laser irradiated surfaces as the ZrO2 transformed to zirconia carbides (ZrC) and Si3N4 to silicon dioxide (SiO2) respectively. The most appropriate equation identified for the determination of the fracture toughness parameter K1c of the as-received, CO2 and the fibre laser surface treated Si3N4 and ZrO2 was K1c=0.016 (E/Hv) 1/2 (P/c3/2). Surfaces of both ceramics treated with CO2 and the fibre laser irradiation produced an increased K1c under the measured conditions, but with different effects. The CO2 laser surface treatment produced a thicker and softer layer whereas the fibre laser surface treatment increased the hardness by only 4%. This is inconsiderable but a reduction in the crack lengths increased the K1c value under the applied conditions. This was through a possible transformation hardening which occurred within both engineering ceramics. Experimental findings validated the generated thermal FEM of the CO2 and the fibre laser surface treatment and showed good agreement. However, a temperature difference was found between the CO2 and fibre laser surface treatment due to the difference in absorption of the near infra-red (NIR) wavelength of the fibre laser being higher than the mid infra-red (MIR) wavelength of the CO2 laser. This in turn, generated a larger interaction zone on the surface that was not induced further into the bulk, as was the case with the fibre laser irradiation. The MIR wavelength is therefore suitable for Viability and Characterization of the Laser Surface Treatment of Engineering Ceramics 3 the surface processing of mainly oxide ceramics and surface treatments which do not require deep penetration. Phase transformation of the two ceramics occurred at various stages during the fibre laser surface treatment. The ZrO2 was transformed from the monoclinic (M) state to a mixture of tetragonal + cubic (T+C) during fibre laser irradiation and from T+C to T and then a partially liquid (L) phase followed by a possible reverse transformation to the M state during solidification. The Si3N4 transformed to a mixture of α-phase and β-phase (α→ α+β) followed by α+β and fully transforms from α+β →β-phase. What is more, is a comparison of the fibre laser-beam brightness parameter with that of the Nd:YAG laser. In particular, physical and microstructural changes due to the difference in the laser-beam brightness were observed. This research has identified the broader effects of various laser processing conditions, as well as characterization techniques, assessment and identification of a method to determine the K1c and the thermal FEM of laser surface treated engineering ceramics. Also, the contributions of laser-beam brightness as a parameter of laser processing and the influence thereof on the engineering ceramics have been identified from a fundamental viewpoint. The findings of this research can now be adopted to develop ceramic fuel cell joining techniques and applications where laser beam surface modification and characterization of engineering ceramics are necessary

    Understanding laser beam brightness: a review and new prospective in material processing

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    This paper details the importance of brightness in relation to laser beams. The ‘brightness’ of lasers is a term that is generally not given much attention in laser applications or in published literature. With this said, it is theoretically and practically an important parameter in laser-material processing. This study is first of a kind which emphasizes in-depth, the concept of brightness of lasers by firstly reviewing the existing literature and the progress with high brightness laser-material processes. Secondly, the techniques used to enhance the laser beam brightness are also reviewed.In addition,we review the brightness fundamentals and rationalize why brightness of lasers is an important concept. Moreover,an update on the analytical technique to determine brightness using the current empirical equations is also provided. A modified equation to determine the laser beam brightness is introduced thereafter. Furthermore, research studies previously conducted to modify laser design to affect laser beam brightness are also discussed. The paper not only involves are view of the techniques used to improve laser beam brightness but also reviews how bright lasers can be employed to enhance/improve laser process capabilities and cost reduction of the laser assisted processes in manufacturing

    Laser surface induced roughening of polymeric materials and the effects on Wettability characteristics

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    It has been thoroughly demonstrated previously that lasers hold the ability to modulate surface properties of polymers with the result being utilization of such lasers in both research and industry. With increased applications of wettability techniques within industries there is greater need of predicting related characteristics, post laser processing, since such work evaluates the effectiveness of these surface treatments. This paper details the use of a Synrad CO2 laser marking system to surface roughen polymeric materials, namely: nylon 6,6; nylon 12, polytetrafluoroethylene (PTFE) and polyethylene (PE). These laser-modified surfaces have been analyzed using 3D surface profilometry to ascertain the surface roughness with the wettability characteristics obtained using a wettability goniometer. From the surface roughness results, for each of the samples, generic wettability characteristics arising from laser surface roughening is discussed

    Modulating the wettability characteristics and bioactivity of polymeric materials using laser surface treatment

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    It has been thoroughly demonstrated previously that lasers hold the ability to modulate surface properties of materials with the result being utilization of such lasers in both research and industry. What is more, these laser surface treatments have been shown to affect the adhesion characteristics and bio-functionality of those materials. This paper details the use of a Synrad CO2 laser marking system to surface treat nylon 6,6 and polytetrafluoroethylene (PTFE). The laser-modified surfaces were analyzed using 3D surface profilometry to ascertain an increase in surface roughness when compared to the as-received samples. The wettability characteristics were determined using the sessile drop method and showed variations in contact angle for both the nylon 6,6 and PTFE. For the PTFE it was shown that the laser surface treatment gave rise to a more hydrophobic surface with contact angles of up to 150° being achieved. For the nylon 6,6, it was observed that the contact angle was modulated approximately ±10° for different samples which could be attributed to a likely mixed state wetting regime. The effects of the laser surface treatment on osteoblast cell and stem cell growth is discussed showing an overall enhancement of biomimetic properties, especially for the nylon 6,6. This work investigates the potential governing parameters which drives the wettability/adhesion characteristics and bioactivity of the laser surface treated polymeric materials

    Influence of laser beam brightness during surface treatment of a ZrO2 engineering ceramic

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    A comparative study between fibre and Nd:YAG (neodymium, yttrium, aluminium, garnet) laser surface treatment on a cold isostatic pressed (CIP) ZrO2 engineering ceramic was conducted to investigate the individual differences of laser brightness (radiance) produced by the two laser sources. The effects of brightness exhibited by the two lasers were investigated in respect to the change in the hardness, dimensional size of the laser radiated zones and the microstructure of the ZrO2 engineering ceramic. The results showed that the hardness of the ZrO2 engineering ceramic was reduced by 36% for the Nd:YAG laser in comparison to the as-received surface. However, only 4% reduction in the surface hardness was found from employing the fibre laser surface treatment which was not significant as much as the results of the Nd:YAG laser radiation. The change in hardness occurred due to softening of the laser radiated surface of the ZrO2 with a changed composition which was softer than the laser unaffected surface. The dimensional size of the fibre laser radiated track was also found to produce broader surface profiles in comparison to that of the Nd:YAG laser. The fibre laser radiated surface track was 32% larger in width and 51.5% longer in depth of penetration in comparison to that of the Nd:YAG laser. Change in microstructure of the ZrO2 engineering ceramic radiated by both lasers was found as opposed to the ground and polished untreated surface with the fibre laser affecting the grain morphology to a greater extent in comparison to that of the Nd:YAG laser radiation. The physical and micro-structural effects from applying the two laser types to the ZrO2 engineering ceramic differed as deep penetration and broader laser radiated track as well as larger grains were produced by the fibre laser, despite using identical laser processing parameters such as spot size, power density, traverse speed, gas flow rate, wavelength and the Gaussian beam profile. This occurred due to the high brightness exhibited by the fibre laser radiation which generated larger power per unit area which in turn induced into the ZrO2 engineering ceramic and resulted to producing high processing temperature, larger fibre laser-ceramic-interaction zone and melt-pool at the laser-ZrO2 interface in comparison to that of the Nd:YAG laser which intrinsically resulted to a change in physical attributes of the ceramic