  Two-dimensional (2D) tungsten disulfide (WS$_2$), tungsten diselenide
(WSe$_2$), and tungsten ditelluride (WTe$_2$) draw increasing attention due to
their attractive properties deriving from the heavy tungsten and chalcogenide
atoms, but their mechanical properties are still mostly unknown. Here, we
determine the intrinsic and air-aged mechanical properties of mono-, bi-, and
trilayer (1-3L) WS$_2$, WSe$_2$ and WTe$_2$ using a complementary suite of
experiments and theoretical calculations. High-quality 1L WS$_2$ has the
highest Young's modulus (302.4+-24.1 GPa) and strength (47.0+-8.6 GPa) of the
entire family, overpassing those of 1L WSe$_2$ (258.6+-38.3 and 38.0+-6.0 GPa,
respectively) and WTe$_2$ (149.1+-9.4 and 6.4+-3.3 GPa, respectively). However,
the elasticity and strength of WS$_2$ decrease most dramatically with increased
thickness among the three materials. We interpret the phenomenon by the
different tendencies for interlayer sliding in equilibrium state and under
in-plane strain and out-of-plane compression conditions in the indentation
process, revealed by finite element method (FEM) and density functional theory
(DFT) calculations including van der Waals (vdW) interactions. We also
demonstrate that the mechanical properties of the high-quality 1-3L WS$_2$ and
WSe$_2$ are largely stable in the air for up to 20 weeks. Intriguingly, the
1-3L WSe$_2$ shows increased modulus and strength values with aging in the air.
This is ascribed to oxygen doping, which reinforces the structure. The present
study will facilitate the design and use of 2D tungsten dichalcogenides in
applications, such as strain engineering and flexible field-effect transistors
(FETs).
