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We report optical and mechanical properties of hard aluminum magnesium boride films magnetron sputtered from a stoichiometric AlMgB₁₄ ceramic target onto Corning^® 1737 Glass and Si (100) wafers. High target sputtering rf-power and sufficiently short target-to-substrate distance appeared to be critical processing conditions. Amorphous AlMgB₁₄ films demonstrate very strong indentation size effect (ISE): exceptionally high nanohardness <i>H</i> = 88 GPa and elastic Young’s modulus <inline-formula><math display="inline"><semantics><mrow><msup><mi>E</mi><mo>*</mo></msup></mrow></semantics></math></inline-formula> = 517 GPa at 26 nm of the diamond probe penetration depth and almost constant values, respectively, of about 35 GPa and 275 GPa starting at depths of about 2–3% of films’ thickness. For comparative analysis of elastic strain to failure index <inline-formula><math display="inline"><semantics><mrow><mo> </mo><mi>H</mi><mo>/</mo><msup><mi>E</mi><mo>*</mo></msup></mrow></semantics></math></inline-formula>, resistance to plastic deformation ratio <inline-formula><math display="inline"><semantics><mrow><msup><mi>H</mi><mn>3</mn></msup><mo>/</mo><msup><mi>E</mi><mrow><mo>*</mo><mn>2</mn></mrow></msup></mrow></semantics></math></inline-formula> and elastic recovery ratio <inline-formula><math display="inline"><semantics><mrow><msub><mi>W</mi><mi mathvariant="normal">e</mi></msub></mrow></semantics></math></inline-formula> were obtained in nanoindentation tests performed in a wide range of loading forces from 0.5 to 40 mN. High authentic numerical values of <i>H =</i> 50 GPa and <inline-formula><math display="inline"><semantics><mrow><msup><mi>E</mi><mo>*</mo></msup></mrow></semantics></math></inline-formula> = 340 GPa correlate with as low as only 10% of total energy dissipating through the plastic deformations.