Laser Treatment of SiC-C 5 Si 3 Ceramics in N 2O 2 Medium

By methods of X-ray diffraction, electron microscopy, atomic force microscopy and X-ray microanalysis, IR-spectroscopy the influence of IR laser irradiation (1064 nm, 175 W) on a target consisting of a SiC-Cr5Si3 ceramic composite was investigated. Irradiation of samples was carried out in N2+ O2 medium at a pressure of 2 at. It was established that the surface temperature is ~ 2000 K. Silicon and silica are evaporated from the surface and during gas medium transit silicon oxide and silicon oxynitride are formed. On the base of ablative products in the zone of precipitation the film was formed.


Introduction
In the analysis of the processes proceeding during laser cutting, thermosplitting, drilling, polishing basic attention was given to radiation interaction of a radiation with substance [1][2][3][4][5][6].Work on studies of physical and chemical processes proceeding in an irradiation zone and resulting in modification of a composite ceramic surface have been developed only recently [7][8].However very little work is devoted to research of composite ceramics on the basis of refractory non-oxygen compounds [10][11].As processes of surface modification of functional composite ceramics result in the formation of new properties of a material, as well as expansion of areas of their application, it is rather important to establish the nature and mechanisms of a surface reorganization in various regimes and conditions of irradiation.
The purpose of the given work is research of a surface modification of SiC-chromium silicide ceramics under the influence of IR-laser radiation in a gas environment, which contains nitrogen with a touch of oxygen.
SiC powder with the addition of Cr and Si counting on formation of Cr 5 Si 3 .The size of samples constitutes: d = 5 mm, l = 10 mm.
Irradiation of samples was done in a nitrogen medium at a pressure of 2 at.with an infrared laser (λ = 1064 nm and P = 175 W).The radius of beam was 2 mm.The velocity of beam movement was 2 mm/c.The content of oxygen in nitrogen was 4 %.During irradiation ablative products were precipitated on a quartz substrate.
An X-ray analysis of samples was performed in a Siemens D-500 diffractometer using Cu K α radiation.Scanning electron microscopy studies were carried out with an HU-200F unit.An X-ray microanalysis of samples was performed in a "Comebax SX50" unit.Atomic force microscopy (AFM) measurements were performed on Nanoscope IV of Digital Instruments.Regimes of height and phase modes were performed.IR-spectra on a Specord M-80 spectrometer were obtained.

Results
According to X-ray phase analysis in ceramics alongside with α-SiC a small amount of β-SiC and chromium silicide (Cr 5 Si 3 ) is present.
From the data of electron microscopy, AFM, and X-ray microanalysis it is possible to conclude that chromium silicide is non-uniformly distributed between SiC grains (Fig. 1).During laser irradiation a rim arises on the border (frontier) with the non-irradiated surface (Fig. 2), which represents a stiffened melt.In an irradiated zone a surface with "heights"is formed (Fig. 3 a): on a background of areas with a smooth relief eminent formations of complex ("lobes") forms (Fig. 3 b) are created.For a higher resolution it is visible that the flat areas represent a porous surface on which grains are incorporated by a stiffened melt (Fig. 3 c).A non-irradiated surface rises over the irradiated area.
Concerning microanalysis the following elements are present on the surface (wt.%): Si (49.8);Cr (28.41);N (14.96);O (5.27); C (1.56).Mainly nitrogen, oxygen and carbon are located in eminent "heights".Chromium is registered in the smoothed and lowered areas of a relief.In them traces of oxygen and nitrogen are registered.The content of carbon does not exceed 1.56 wt.%.
In the IR-spectrum of ablative product bands of absorption, characteristic for silicon nitride both films of silicon oxide and silicon oxynitride (see Table I and Fig. 4) are registered.In the region 1500 -1700 cm -1 absorption bands of N = O bonds of adsorbed gases on a sample surface are registered [9].The precipitated ablative products are represented by nano-particles aggregates.The size of particles is between 7-10 nm.They form aggregates with sizes from 30 nm up to 430 nm (Fig. 5).
Tab.I IR-bands of absorption observed in the range 400 -2000 cm -1 in SiO

Discussion
In [10,11] it was established that laser irradiation of a SiC -Cr 5 Si 3 ceramic surface with λ = 1064 nm in air is accompanied by increased surface temperatures to 2000 K.At that silicon evaporates from SiC and a superficial layer is enriched with carbon.Chromium silicide melts and covers the whole of the surface.With an increased time irradiation chromium silicide oxidizes and chromium and silicon oxides enrich the sample surface.SiO 2 and Cr 2 O 3 do not give glass-like films and silica starts to evaporate.
In the case of laser irradiation in a nitrogen medium all processes, described above, due to the deficiency of oxygen develop weakly.For this reason, we have the possibility to observe what occurs with the melt under laser beam exposure.Formation of a rim indicates that part of the melt "splashes out" (effluent).This is typical for laser cutting of metals and glasses and is accompanied by sublimation-evaporation of irradiated materials [1][2][3][4][5][6].
As the composite ceramics includes SiC grains, at the lowering of the melt level similarly to "islands" they appear above the surface of the stiffened chromium silicide melt.At T ~ 2000 K the reaction SiC = Si g + C s takes place, where g is a gaseous substance, s is a solid state one.Allocated silicon during its flight through the gas environment, containing nitrogen and oxygen, is transformed into silicon nitride, silicon oxynitride and silicon oxide.
In turn, mitrogen and oxygen can diffuse into superficial layers of silicon carbide grains and the chromium silicide melt.
Liberation of CO g (CO 2 ) g (as a result of C + O 2 reaction) and gassing during chromium silicide melting is a source of porous formation in a stiffened melt.
Detection of carbon in places of chromium silicide localization does not exclude formation of chromium carbosilicide at its melting.
The common schema of a surface modification is presented on Fig. 6.

Fig. 1
Fig. 1 Electron micro photo of SiC + 10 % wt.Cr 5 Si 3 ceramics according to electron microscopy (a) and atomic force (b) data.The identification phase on (b) was made with respect to electron microscopy and X-ray microanalysis.

Fig. 2
Fig. 2 Surface area of the irradiated ceramic.(a) according to electron microscopy and (b,c) AFM data in the region of a rim face.(b) exposition in the height regime; (c) exposition in the phase regime.

Fig. 3
Fig. 3 Micro photo of a ceramic surface area in the zone of irradiation.
-strong, m -medium, w -weak intensity of the adsorption band, sh-shoulder on the main band.

Fig. 5
Fig. 5 AFM data of ablative products precipitated in nitrogen.(a) exposition in the height

Fig. 6 A
Fig. 6 A common schema of surface modification during laser irradiation.(I) ceramic; (II) initial stage of ceramic laser irradiation; (III) final stage of ceramic laser irradiation.
1.During laser irradiation of SiC + Cr 5 Si 3 composite ceramics heating of the surface up to T ~ 2000 K occurs.Садржај: