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【正文】 initially high, as a reasonable consequence of the presence of an outer, friable region constituted by disordered slope progressively decreased down to a stable value of about _mkm?1. Under 25N of load (Fig. 6b), the behaviour of the borided steel was again intermediate between those of the nitrided samples and WC–Co coated samples, while the behaviour of hard chromium was parable to that displayed by the nitrided steel. The wear–distance curve of the borided steel was initially steep, as expected. Then the slope decreased, possibly because of the higher wear resistance of ordered boride crystals, and finally it increased considerably up to values in excess of 43 _mkm?1 when, because of wear, unborided iron zones came in contact with the counterfacing ceramic material. The friction coefficient μ of the borided steel, initially low for the low shear strength displayed by the outer, disordered region of the coating, gradually increased to values significantly fluctuating around ～, a value parable with those reported in the literature for coatings sliding against steel and other engineering alloys [11,12]. It is worth noting that a substantial decrease in the friction coefficient from to less than was obtained for a borided lowcarbon steel sliding in air at room temperature against a Si3N4 ball under 5N and 2–4mms?1, by exposing the boride coating to air atmosphere at 750 ?C for 3 min [11]. The improvement was explained as the effect of reaction between oxygen in air and boron in the coating during the hightemperature exposure: first a film of boron oxide formed which, subsequently, reacted with moisture in air giving rise to a lubricious film of boric acid. A more detailed evaluation of the tribological behaviour of boride coatings was obtained by testing their resistance to abrasive wear. As shown in Fig. 7, in fact, the wear rate expressed as volume of material worn per distance unit and applied load unit was very high for asborided samples (pt. 1 in Fig. 7), due to the already mentioned presence of an outer, mechanically friable region in the boride coating. The rate was considerably lower for inner regions (pt. 2 in Fig. 7) that, on the basis of metallographic observations, were found to be constituted by FeB and, to a less extent, by Fe2B. On going further inwards through the coating, the wear rate increased for regions around the FeB–Fe2B interface (pt. 3 in Fig. 7), and became minimum for the regions constituted by highly ordered Fe2B crystals (pt. 4 in Fig. 7). Then, the wear rate increased for regions constituted by Fe2B columns and unborided iron (pt. 5 in Fig. 7) and, even more, when only pure, soft iron was submitted to abrasion (pt. 6 in Fig. 7). The tribological behaviour pointed out by the wear rate values reported in Fig. 7 confirms the necessity to submit borided ponents to a surface finishing procedure suitable to remove the outer, poorly resistant part of the boride coating. Moreover, it may well explain why for several applications a single phase coating of ordered and tough Fe2B is preferred to a polyphase boride coating, harder but prone to premature failures at the highly stressed FeB–Fe2B interface. 4 Conclusions The polyphase boride coatings thermochemically grown on iron and constructional steel are constituted by an outer layer of FeB and an inner layer of Fe2B, with an extent of crystallographic order that is considerably different for regions located at different depths from the external surface. Mechanical pactness and wear resistance depend on this order. The wear rate of the coatings was initially high, both under sliding and abrasive conditions, because of the presence of an outer, thin and friable layer of disordered crystals. Then the wear rate decreased because of the resistance offered by ordered crystals first of FeB and then of Fe2B, until it reached a minimum value in the inner, very pact regions of the coatings constituted by highly ordered crystals of Fe2B. The tribological behaviour of polyphase boride coatings can be worsened by intense stress states at the FeB–Fe2B interface, which can lead to nucleation and propagation of cracks along the interface, with formation of hard and abrasive wear fragments. Measurements of wear rate carried out through the thickness by means of the ballcratering and layerbylayer methods proved to be effective for evaluating the wear resistance of polyphase coatings at different depths. 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