The flattening ratio, ε, is an important diagnostic measurement of particle deformability and coating quality assessment in cold spray. However, the experimental determination of ε and correlation of it to experimental parameters (particle size, velocity) is not straightforward from observations of cold-sprayed coatings. Here we present a series of direct site-specific measurements of ε for well-known particle sizes and velocities, without the many complications involved in a full cold spray experiment. This approach permits us to identify mechanistic transitions separating three apparent regimes of behavior. In regime I, at lower velocities just high enough to cause bonding, an impacting particle flattens largely unhindered. In regime II, at somewhat higher velocities, the substrate develops a significant crater, which in turn constrains lateral particle flattening as the particle burrows more deeply into the substrate. In regime III, at very high velocities, hydrodynamic particle penetration is observed, distinguished by deep burrowing of the particle, which adopts a concave-saucer shape, with large petals of substrate ejected backwards out of the surface. While direct measurements in regimes I and II show that deformed particles can be reasonably modeled as oblate spheroids, that assumption fails in regime III. To the extent that particle flattening is deemed desirable, this analysis suggests a more nuanced view of the velocities used in cold spray.

扁平率ε是冷喷涂中颗粒变形性和涂层质量评估的重要诊断指标。然而，ε的实验确定及其与实验参数（粒度，速度）的关系从冷喷涂涂层的观察中并不容易。在这里，我们介绍了一系列针对已知粒径和速度的ε直接针对特定地点的测量结果，而没有涉及整个冷喷涂实验的许多复杂情况。这种方法使我们能够确定将三种明显的行为方式分开的机械转换。在方案I中，较低的速度刚好足以引起粘结，因此撞击粒子在很大程度上不受阻碍地变平。在方案II中，在较高速度下，基材会形成明显的弹坑，当颗粒更深地钻入基材时，这又限制了颗粒的横向展平。在方案III中，在很高的速度下，观察到流体动力学的颗粒渗透，其特征是颗粒深陷，呈凹形碟形，基片大片向后弹出。虽然在方案I和II中进行的直接测量表明，变形颗粒可以合理地建模为扁球体，但在方案III中这一假设失败了。在认为颗粒变平的程度上，该分析表明对冷喷涂中使用的速度有更细微的了解。其特点是颗粒呈深凹状，呈凹刀盘状，大片基片花瓣从表面向后弹出。虽然在方案I和II中进行的直接测量表明，变形颗粒可以合理地建模为扁球体，但在方案III中这一假设失败了。在认为颗粒变平的程度上，该分析表明对冷喷涂中使用的速度有更细微的了解。其特点是颗粒呈深凹状，呈凹刀盘状，大片基片花瓣从表面向后弹出。虽然在方案I和II中进行的直接测量表明，变形颗粒可以合理地建模为扁球体，但在方案III中这一假设失败了。在认为颗粒变平的程度上，该分析表明对冷喷涂中使用的速度有更细微的了解。