| Summary: | 碩士 === 逢甲大學 === 材料科學所 === 90 === It is well known that a duplex treatment combining ion nitriding and ceramic hard coating can significantly enhance the wear performance and lifetime of tool steel due to the promoted load-carrying-capacity. Extensive researches on this issue have been done with however shortage of the lack of understanding the effect of pre-nitriding layer structure and the bombarding temperature that used during ceramic top coating on the mechanical properties. To deal with this, it necessitates a systematically study.
An attempt is to control the structure of the nitriding layer of two commonly used high speed steel and hard coatings were deposited at different bombarding temperature to confirm the conditions of denitriding and the mechanical properties for above-mentioned arrangements.
Experimental result shows that SKH 51 and SKH 2 have similar diffusion layer formation at the 10% nitriding atmosphere. Single γ´ phase or ε phase could be obtained at 33% nitriding atmosphere or ε phase at 83% nitriding atmosphere plus CH4. SKH 51 exhibits higher nitriding efficiency and nitriding layer was thicker. In XRD pattern, the TiN coating preferred (111) orientation on all different nitrided and ion-bombarded substrates. This demonstrates that coating structure would not be affect by duplex treatment conditions.
After all bombarding temperature treatment, black layer was not observed on α solid solution and γ´-Fe4N. But it appears on two high speed steels with ε-Fe3N. Black layer in SKH 51 specimen is thinner than other kinds of specimens at all bombarding temperature. Oppositely, black layer in SKH2 specimen formed above lower temperature of 400℃, it reveal that the compound layer in SKH 51 is more stable than it in SKH 2. This may due to the nitrogen uptake by the nitride former elements in the steels. From the compositional depth-profile, nitrogen depth-profile decreases with the increase of bombarding temperature. In medium temperature, the titanium-rich layer between nitriding layer and TiN top coating is formed by titanium bombarding flux before TiN coating. In high temperature, the nitrogen elements of the nitriding layer diffused outward and react with Ti-rich layer to form TiN.
Because the nitriding layer of SKH 51 is thicker, the surface hardness of SKH 51 specimen is little higher than SKH 2, the hardness differences of duplex-treated specimen are not significant. It was Mainly influenced by different nitriding structures and the highest hardness is the phase of ε-Fe3N after duplex treatment. Bombarding temperature also has extreme influence the surface hardness of duplex treatment. When bombarding temperature at 450℃, all of the duplex treatment exhibits the highest hardness. If bombarding temperature is higher than 500℃, the substrate of SKH 51and SKH 2 will become softer. The hardness is independent on black layer.
From scratch test results, the scratch load of TiN coating on duplex treatment at low bombarding temperature is over the test limit 100 N, so the substrate material effect of adhesion could not be compared. As bombarding temperature increasing, this is caused by temper soften of substrate. In three nitriding structures, it is found that ε-Fe3N provides the best coating adhesion, which could be concluded that the adhesion of the TiN coating would not be affected by forming the black layer.
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