The Thermochemical Surface Modification of the Iron-Carbon Alloys
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267 Mat. Res. Soc. Symp. Proc. Vol. 555 © 1999 Materials Research Society
EXPERIMENT The coatings under treatment were prepared on the surface of irons and steel (FeC alloys). The samples have been exposed to overheated vapor of water solution of salts. Concentration of salt was 0.01-10g/l, the vapor temperature varied from 450 to 620C. For the comparison it was used overheated water vapor treatment. All exposures have been done in the closed stain steel chamber. Optical microscope as well as SEM (JEOL 820) were used to monitor the structure of coatings. To obtain the element concentration profiles we've used SEM with EDS. Chemical composition of surface layers have been studied with XPS ( KRATOS XPS 800). Phase composition was evaluated with X-ray diffraction technique (XRD). RESULTS Structure and morpholoev, comoosition As we discussed in introduction, one can expect the non-uniform multilayer structure of the coating deposited from overheated vapor. That's why we start description with SEM and optical microscope. Typical SEM image of the substrate after treatment in water (NH 2)4MoO 4 + (NH 4)2 S20 3 vapor is presented at Fig. 1. To obtain this image original sample was broken in the direction perpendicular to surface. The substrate (gray iron FeC with small amount of Si, Mn, Ni, Cr, S) is the lowest layer seen. The next upper layer (I) which is the closest to substrate has certainly another morphology. XRD shows the small and highly dispersive grain size. SEM + EDA analysis brings up that this layer consist of FeO mixed with oxidized doping elements of substrate (Si, Mn, Ni, Cr, S).
III II
Fig. 1 Image of modified layer (magnification 900)
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Corresponding depth profiles of the constituting elements for the same sample are presented at Fig.2. The next layer (Ii) has well-defined vertical column-like crystalline structure. It has the most complicated element composition. Both elements from saturating vapor and from substrate are mixed in this region (Fig.2). The uppermost layer (III) has large grain size morphology with the uniform axial distribution. It was condensed mainly of substances of saturating vapor, and thus its composition measured with XPS reflects the element distribution of vapor (Table 1).
l0ow
500 750
250
20
E
10
,0a) c
P
50 25,
50.
25.
75.
I 25
0
10
20
30
100
40
Thickness, microns
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Fig.2. Profiles of element Xray irradiation intensity of modified layer on iron surface.
Tablel. Composition of upper part of modified layer Saturating environment composition H20 + (NH4 )2MoO 4
H20
+ KNaCrO 4
H20 +NiSO 4
Salt concentration, g/l
Surface layer composition
3.0- 5.0
FeMoO 4, FeO, (NHa) 2Mo 20 7 , MoO 2, FeSiO 2 FeO, FeCr 20 4, FeSiO 2, K2Cr 2 07 FeSiO 2, NiMn0 4, FeO, NiSO 4, FeMnO4
10.0
-
12.0
5.0 - 7.0
T=600C, t = 40min. Mechanical properties This picture of three-layer formation is very similar for all samples studied with variation of thickness of layers. For the discussion of tribo characteristics we have to notice that these layers have different mecha
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