February AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 9 2 2 NOVEL MAB PHASE-BASED NANOLAMINATES SUIT HIGH- PERFORMANCE APPLICATIONS The unique structure of MAB-phase ternary borides offers outstanding material properties. Surojit Gupta and Maharishi Dey University of North Dakota, Grand Forks T he need for high performance ma- terials for use in aerospace appli- cations is driving the search for novel structural materials. Refractory metal diborides (e.g., zirconium diboride, ZrB 2, and hafnium diboride, HfB 2 ) have emerged as possible candidates for use in high performance applications such as cutting tools, leading edges for hy- personic vehicles, and high temperature electrodes. However, some limitations of these materials include brittleness, low fracture toughness, low electrical con- ductivity, and low oxidation resistance. The addition of SiC to refractory metal borides improves oxidation resistance, but the mechanical and physical prop- erties of the composites depend on the SiC concentration. A more fundamental route to improving properties is to tai- lor the crystal structure of borides. For example, adding stable, protective ox- ide-forming elements like aluminum into the crystal lattice produces refractory ter- nary borides. Ade and Hillebrecht [1] integrated different compositions and crystal struc- tures of boride-containing ternaries in- to the chemical formula (MB) 2 Al y (MB 2 ) x , classifying them as MAB phases due to their two dimensionality and com- bination of covalent and metallic inter- actions. These are reminiscent of MAX phases, ternary carbides and nitrides with the chemical formula M n+1 AX n , where M is an early transition metal, A is a group A element, and X is either C or N. MAX phases have layered nano- laminate structures where MX 6 oc- tahedra are interleaved with group A elements [2-5] . Neguib et al. [6] demon- strated that it is possible to design 2D materials from MAX phases by exfolia- tion of Ti 3 AlC 2 by HF at room tempera- ture, and classified them as MXenes. LiF/HCl is the common etchant for fabri- cating MXenes fromMAX phases by gen- erating HF in situ by reaction [7] . POTENTIAL OF MAB PHASES The unique structure of MAB pha- ses offers outstanding material prop- erties [1] . Several researchers have syn- thesized MoAlB, Mn 2 AlB 2 , and Fe 2 AlB 2 in single phase dense form [8-11] . Table 1 summarizes the properties of these solids compared with MAX phases like Ti 3 SiC 2 and Ti 3 AlC 2 . In addition, the fracture tough- ness of MoAlB and Fe 2 AlB 2 are 4.3±0.1 and 5.4±0.2 MPa m 1/2 , respectively [9,11] . Ternary borides are excellent conductors of heat and electricity like MAX phases, but they have higher strength and hardness. The properties of ternary borides fall between soft carbides and nitrides like MAX phases and binary hard borides, thus offering a novel material system for use in de- manding applications. MoAlB and its particulates potentially can be used in tribo- active and temperature sensitive appli- cations [10,12] . MAB phase-based particu- lates can also be used as templates to design particles with a tailored chemis- try and periodically etched surfaces, or 2D phases [13-15] . CASE STUDY: DESIGNING ETCHED MoAlB PARTICULATES AND COMPOSITES Figure 1 shows the XRD pattern of 0.37 mol of ball milled MoAlB particu- lates treated with 12 M HCl etchant, and LiF/HCl molar ratios of 0.26 and 1.03, respectively. All compositions showed MoAlB as the major crystalline phase although the intensity of MoB peaks in- creases in the etched composition. This study indicates that MoAlB is stablewith the etchants used together with some Fig. 1 — XRD patterns of MoAlB particulates: (a) milled MoAlB; (b) MoAlB-12M HCl; (c) MoAlB-12 M HCl-0.1M LiF; and (d) MoAlB-12M HCl-0.37M LiF.