Laser carbonitriding of alumina surface

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Abstract

Laser carbonitriding of alumina surfaces is examined. Temperature and stress fields developed during the laser heating of the substrate surface are predicted using the finite element method in line with the experimental conditions. The formation of Al(C, N) and AlN compounds in the surface region of irradiated workpiece is examined using X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD). The microstructural and morphological changes in the laser irradiated region are examined using Scanning Electron Microscope (SEM). The microhardness of the resulting surface is measured and compared with the base material hardness. It is found that high temperature gradient is developed in the irradiated region, which in turn, results in high residual stress levels in this region. XPS and XRD data reveal the presence of Al (C, N) and AlN compounds in the surface region. The microhardness in the surface region of the laser treated workpiece increases significantly.

Research highlights

► The laser controlled melting of alumina ceramics improves the surface properties through homogenizing the microstructure in the surface region, transformation of metastable γ-phase into equilibrium α-phase, and other chemical transformations including nitriding. ► In the present study, laser carbonitriding of alumina surfaces is examined. ► Temperature and stress fields developed during the laser heating of the substrate surface are predicted using the finite element method in line with the experimental conditions. ► The formation of Al(C, N) and AlN compounds in the surface region of irradiated workpiece is examined using X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD). ► The microstructural and morphological changes in the laser irradiated region are examined using Scanning Electron Microscope (SEM).

Introduction

Alumina ceramics find wide applications in electronic and biomedical industries. However, the laser controlled melting of alumina ceramics improves the surface properties through homogenizing the microstructure in the surface region, transformation of metastable γ-phase into equilibrium α-phase, and other chemical transformations including nitriding [1]. The laser controlled melting involves with the solid heating, melting and superheating of liquid phase. Since the laser beam scans the surface at a constant speed, the high cooling rates result in excessive thermal strains and stresses development in the irradiated region. The thermally induced cracks and high residual stress levels are formed in the irradiated region for some laser processing parameters. Moreover, in laser processing, an assisting gas is used to prevent the surface from the high temperature oxidation reactions. In this case, in general, nitrogen is used as an assisting gas to prevent the oxidation reactions. Moreover, it was demonstrated that the presence of high pressure nitrogen gas in the irradiated region results in the formation of nitride species [1]. This, in turn, improves the hardness of the laser treated alumina surfaces. However, the presence of carbon film on the alumina surface during the laser gas assisted nitriding can cause the formation of the carbonitride species in the surface region of the irradiated substrate material. Consequently, investigation into laser gas assisted carbonitriding of alumina surfaces and the development of the thermal stresses in the irradiated region becomes necessary.

Considerable research studies are carried out to examine the laser surface melting of alumina. Laser treatment of alumina surfaces results in changes in the microstructure formed at the irradiated surface during the laser re-melting process [2]. Some of these changes include the formation of column-like crystals [3] and dense structures with reduced grain sizes, which improve the wear resistance of the surface [4]. The fractal dimensions of the surface microstructure can be correlated with the surface features of the laser heated alumina [5]. The microhardness enhances after the laser re-melting process because of the high residual stress formation in the surface region; in which case, the microcracks formation in the irradiated surface becomes unavoidable due to the high temperature gradient developed in the surface region during the solidification [6], [7], [8], [9], [10]. The laser power intensity and the laser wavelength are the influencing parameters on the structure of the resulting surface; in which case, the microcracks could be avoidable using low power and short wavelength lasers such as diode lasers [11]. However, the depth of the laser treated layer becomes shallow at low laser power intensities [12]. The thin layer of plasma electrolytic oxidation is found to be effective in producing dense morphology with minimal stress intensity and relatively high thermodynamically stable α-Al2O3 phase in the surface region [13], which enhances the wear resistance of the surface [9]. In addition, the surface plasma formed during the laser treatment process has effects on the nitrogen diffusion into the alumina surface and the growth rate of AlN in the laser treated region [14], [15].

In the previous study [1], laser gas assisted nitriding of alumina surfaces was investigated. However, the further improvement of the surface properties, such as hardness, can be possible through carbonitriding of the surface. Consequently, in the present study, laser gas assisted carbonitring of alumina surfaces is carried out. Temperature and stress fields developed in the irradiated region are simulated using the finite element method. The microstructural and morphological changes in the laser assisted carbonitrided regions are examined using Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-ray diffraction (XRD). The microhardness of the resulting surface was also examined.

Section snippets

Experimental

The CO2 laser (LC-ALPHAIII) delivering nominal output power of 2 kW was used to irradiate the workpiece surface. The nominal focal length of the focusing lens was 127 mm employed. The laser beam diameter focused at the workpiece surface will be ∼0.9 mm. Nitrogen assisting gas emerging from the conical nozzle and co-axially with the laser beam will be used. The large range of laser treatment parameters were incorporated in the initial laser treatment tests; the range of parameters resulting in

Heating, thermal stress, and diffusion analysis

In the analysis, the solid body heat conduction with temperature-dependent conductivity, internal energy (including latent heat effects), and convection and radiation boundary conditions are considered. The Fourier heat transfer equation pertinent to the laser heating process can be written asρDEDt=((kT))+Sowhere E is the energy gain by the substrate material, k the thermal conductivity, and So the heat source term resembling the laser beam, i.e.So=Io(1rf)e((x2+y2)/a2)Io is the laser power

Numerical simulation

Finite element discretization was carried out using the ABAQUS software [17]. The simulation is performed in ABAQUS/Standard and consists of sequential thermal stress analysis. In the sequential thermal stress analysis, 91572 elements are used to create the model using two element types; for the heat transfer analysis, mesh used elements of type DC3D4 (4-node Linear heat transfer tetrahedron) and stress analysis used C3D4 (4-node Linear 3D stress tetrahedron). Fig. 1(b) shows the mesh used in

Results and discussion

Laser surface carbonitriding of alumina tiles is investigated. Temperature and stress fields after the laser processing are predicted in line with the experimental parameters. Metallurgical and morphological changes in the laser irradiated region are examined using SEM, EDS, XRD and XPS.

Fig. 2 shows temperature distribution along the x-axis (laser scanning direction) for different cooling periods while Fig. 3 shows temperature contours after 0.05 s of the heating duration. It should be noted

Conclusion

Laser carbonitriding of alumina surface is carried out. Temperature and stress fields developed in the laser heated region are computed using the finite element method in line with the experimental conditions. The microstructural and morphological changes in the laser irradiated region are examined using the SEM, EDS, XRD and XPS. It is found that temperature well exceeds the melting temperature of the substrate material in the irradiated region. The superheating of the liquid phase in the

Acknowledgement

The authors acknowledge the support of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia for the funded project, Project # SB100011.

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