【正文】 iron borides strengthens on going inwards through the polyphase coating. In particular, the inner part of the Fe2B sublayer is constituted by boride crystals all arranged with their [0 0 1] crystallographic axis oriented perpendicularly to the surface of the sample. A specific study has been recently addressed to explain the occurrence of the preferred orientation which characterizes the iron borides thermochemically grown on iron and steels [10]. The conclusions were that the first crystals of Fe2B grow on the external surface of the sample, starting from the contact zones between metal surface and boronising particles, with an acicular form because of the existence of an easiest [0 0 1] direction of growth, coincident with the easier path for boron diffusion in the body centred tetragonal lattice of Fe2B. The growth of boride crystals meets with an increasing number of obstacles as the surface covering increases. Consequently, an increasing number of boride crystals is forced to grow inwards, a more difficult growth path because of the increase in volume associated to formation of borides. The inwards growth of Fe2B is easier if the elongated boride crystals grow parallel. As a consequence, the new boride crystals forming at the coating–metal interface are progressively forced to grow with their [0 0 1] axis perpendicular to the external surface, . aligned to the boron gradient in the sample, giving rise to a (002) Fe2B preferred orientation which progressively strengthens on going towards the interface with the metal substrate [10]. Fig. 6. Wear curves for borided iron and, for parison, curves for steel samples submitted to alternative surface treatments, at different values of applied load: (a) 5N and (b) 25 N. Fig. 7. Wear rates determined by the ballcratering method at different depths from the external surface of a polyphase boride coating grown on iron, after layerbylayer removal of material. Fig. 6 shows the wear behaviour of the borided steel under dry sliding conditions, in parison with samples submitted to alternative surface modifications. Under 5N of load (Fig. 6a), the behaviour was intermediate between those of nitrided steel (worse) and WC–Co (best), and parable to that displayed by hard chromium. The slope of the wear–distance curve for the polyphase coating was 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. Me