【導讀】根據(jù)滑動和磨損試驗條件對樣本的硼摩擦學行為進行了研究。區(qū)的涂料的磨損率有很大的不同。鐵硼化物晶體秩序解釋了這些差異的原因。涂料層構成的無序晶體對兩種類型的抗磨損行為影響不大。然后，構成Fe2B單緊湊，高。度有序的晶體的地區(qū)阻力增加到最高值。耐干滑動樣本的硼優(yōu)于通過提交替代表面處理樣。本資料和含量較低的aWC鈷硬質合金涂層。在這重要的熱化學處理的鋼種擴散的領域，如碳、氮和硼，硼處于一種特殊地位。值得注意的是，最好的結果是由粘固獲得，即工序使用含有粉末的混合物進行硼。不適合過程控制和自動化，這種狀況妨礙了充分傳播滲硼處理工藝。然而，獲得單相層需要滲硼熱處理，即涂層由含量較少但脆性不大的硼化物Fe2B單。僅由未經(jīng)處理的樣。損害的磨損機制，特別是涂層內鐵的硼化物晶體取向的作用。另一方面，有人指出，最外層幾微米的硼化物涂層厚度地。本工作的目的是調查對鋼鐵。C滲硼15小時并使用粉末混合組成的碳化硼

　　

【正文】 s 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. Acknowledgements The authors wish to thank Mr. Andrea Galassi, Institute of Metallurgy, Italy, for the support given in microscale abrasion testing. This work was carried out with financial support from the Ministry of Education, University and Research (MIUR), Rome, Italy. References [1] . Sinha, Boriding (boronizing), in: Heat Treating (Section: Surface Hardening of Steel), vol. 4, ASM Handbook, 1999, pp. 437–447. [2] R. ChatterjeeFischer, Boriding and diffusion metallizing, in: . Sudarshan (Ed.), Surface Modification Technologies, Marcel Dekker, New York, 1989, Chapter 8, pp. 567–609. [3] K. Bartsch, A. Leonhardt, Surf. Coat. Technol. 116–119 (1999) 386. [4] E. Rodr180。?guez Cabeo, G. Laudien, S. Biemer, . Rie, S. Hoppe, Surf. Coat. Technol. 116–119 (1999) 229. [5] A. K252。per, X. Qiao, . Stock, P. Mayr, Surf. Coat. Technol. 120 (2020) 87. [6] M. Carbucicchio, G. Palombarini, J. Mater. Sci. Lett. 6 (1987) 1147. [7] G. Palombarini, M. Carbucicchio, J. Mater. Sci. Lett. 6 (1987) 415. [8] . Rutherford, . Hutchings, J. Testing Eval. 3 (1997) 250. [9] . Staia, . Enriquez, . Puchi, . Lewis, M. Jeandin, Surf. Eng. 14 (1998) 49. [10] C. Martini, G. Palombarini, M. Carbucicchio, J. Mater. Sci. 39 (2020) 1. [11] C. Bindal, A. Erdemir, Appl. Phys. Lett. 68 (1996) 923. [12] R. Iakovou, L. Bourithis, G. Papadimitriou, Wear 252 (2020) 1007.